Did u try the amdgpu-pro driver as well?
i just did four runs of glmark and it just went through for me. going up to
1600mhz shader clock. tested both closed and opensource drivers. vega pulse
here.

mesa result:

=======================================================
    glmark2 2014.03
=======================================================
    OpenGL Information
    GL_VENDOR:     X.Org
    GL_RENDERER:   Radeon RX Vega (VEGA10, DRM 3.33.0, 5.3.1-arch1-1-ARCH, LLVM
8.0.1)
    GL_VERSION:    4.5 (Compatibility Profile) Mesa 19.1.7
=======================================================
[build] use-vbo=false: FPS: 8617 FrameTime: 0.116 ms
[build] use-vbo=true: FPS: 10534 FrameTime: 0.095 ms
[texture] texture-filter=nearest: FPS: 11214 FrameTime: 0.089 ms
[texture] texture-filter=linear: FPS: 11274 FrameTime: 0.089 ms
[texture] texture-filter=mipmap: FPS: 10197 FrameTime: 0.098 ms
[shading] shading=gouraud: FPS: 9790 FrameTime: 0.102 ms
[shading] shading=blinn-phong-inf: FPS: 10979 FrameTime: 0.091 ms
[shading] shading=phong: FPS: 10167 FrameTime: 0.098 ms
[shading] shading=cel: FPS: 9662 FrameTime: 0.103 ms
[bump] bump-render=high-poly: FPS: 9830 FrameTime: 0.102 ms
[bump] bump-render=normals: FPS: 10151 FrameTime: 0.099 ms
[bump] bump-render=height: FPS: 10870 FrameTime: 0.092 ms
libpng warning: iCCP: known incorrect sRGB profile
[effect2d] kernel=0,1,0;1,-4,1;0,1,0;: FPS: 12008 FrameTime: 0.083 ms
libpng warning: iCCP: known incorrect sRGB profile
[effect2d] kernel=1,1,1,1,1;1,1,1,1,1;1,1,1,1,1;: FPS: 10876 FrameTime: 0.092
ms
[pulsar] light=false:quads=5:texture=false: FPS: 10232 FrameTime: 0.098 ms
libpng warning: iCCP: known incorrect sRGB profile
[desktop] blur-radius=5:effect=blur:passes=1:separable=true:windows=4: FPS:
6842 FrameTime: 0.146 ms
libpng warning: iCCP: known incorrect sRGB profile
[desktop] effect=shadow:windows=4: FPS: 7934 FrameTime: 0.126 ms
[buffer]
columns=200:interleave=false:update-dispersion=0.9:update-fraction=0.5:update-method=map:
FPS: 1770 FrameTime: 0.565 ms
[buffer]
columns=200:interleave=false:update-dispersion=0.9:update-fraction=0.5:update-method=subdata:
FPS: 2308 FrameTime: 0.433 ms
[buffer]
columns=200:interleave=true:update-dispersion=0.9:update-fraction=0.5:update-method=map:
FPS: 1875 FrameTime: 0.533 ms
[ideas] speed=duration: FPS: 4475 FrameTime: 0.223 ms
[jellyfish] <default>: FPS: 9499 FrameTime: 0.105 ms
[terrain] <default>: FPS: 2593 FrameTime: 0.386 ms
[shadow] <default>: FPS: 9423 FrameTime: 0.106 ms
[refract] <default>: FPS: 6008 FrameTime: 0.166 ms
[conditionals] fragment-steps=0:vertex-steps=0: FPS: 11364 FrameTime: 0.088 ms
[conditionals] fragment-steps=5:vertex-steps=0: FPS: 10816 FrameTime: 0.092 ms
[conditionals] fragment-steps=0:vertex-steps=5: FPS: 12000 FrameTime: 0.083 ms
[function] fragment-complexity=low:fragment-steps=5: FPS: 10932 FrameTime:
0.091 ms
[function] fragment-complexity=medium:fragment-steps=5: FPS: 11690 FrameTime:
0.086 ms
[loop] fragment-loop=false:fragment-steps=5:vertex-steps=5: FPS: 11119
FrameTime: 0.090 ms
[loop] fragment-steps=5:fragment-uniform=false:vertex-steps=5: FPS: 11003
FrameTime: 0.091 ms
[loop] fragment-steps=5:fragment-uniform=true:vertex-steps=5: FPS: 12886
FrameTime: 0.078 ms
=======================================================
                                  glmark2 Score: 9119 
=======================================================

amdgpu-pro result:

=======================================================
    glmark2 2014.03
=======================================================
    OpenGL Information
    GL_VENDOR:     ATI Technologies Inc.
    GL_RENDERER:   Radeon RX Vega
    GL_VERSION:    4.6.13572 Compatibility Profile Context
=======================================================
[build] use-vbo=false: FPS: 3727 FrameTime: 0.268 ms
[build] use-vbo=true: FPS: 9516 FrameTime: 0.105 ms
[texture] texture-filter=nearest: FPS: 7346 FrameTime: 0.136 ms
[texture] texture-filter=linear: FPS: 9236 FrameTime: 0.108 ms
[texture] texture-filter=mipmap: FPS: 9161 FrameTime: 0.109 ms
[shading] shading=gouraud: FPS: 9184 FrameTime: 0.109 ms
[shading] shading=blinn-phong-inf: FPS: 9363 FrameTime: 0.107 ms
[shading] shading=phong: FPS: 9424 FrameTime: 0.106 ms
[shading] shading=cel: FPS: 9060 FrameTime: 0.110 ms
[bump] bump-render=high-poly: FPS: 9047 FrameTime: 0.111 ms
[bump] bump-render=normals: FPS: 8804 FrameTime: 0.114 ms
[bump] bump-render=height: FPS: 9156 FrameTime: 0.109 ms
libpng warning: iCCP: known incorrect sRGB profile
[effect2d] kernel=0,1,0;1,-4,1;0,1,0;: FPS: 9121 FrameTime: 0.110 ms
libpng warning: iCCP: known incorrect sRGB profile
[effect2d] kernel=1,1,1,1,1;1,1,1,1,1;1,1,1,1,1;: FPS: 8866 FrameTime: 0.113 ms
[pulsar] light=false:quads=5:texture=false: FPS: 8286 FrameTime: 0.121 ms
libpng warning: iCCP: known incorrect sRGB profile
[desktop] blur-radius=5:effect=blur:passes=1:separable=true:windows=4: FPS:
3789 FrameTime: 0.264 ms
libpng warning: iCCP: known incorrect sRGB profile
[desktop] effect=shadow:windows=4: FPS: 4491 FrameTime: 0.223 ms
[buffer]
columns=200:interleave=false:update-dispersion=0.9:update-fraction=0.5:update-method=map:
FPS: 1026 FrameTime: 0.975 ms
[buffer]
columns=200:interleave=false:update-dispersion=0.9:update-fraction=0.5:update-method=subdata:
FPS: 2228 FrameTime: 0.449 ms
[buffer]
columns=200:interleave=true:update-dispersion=0.9:update-fraction=0.5:update-method=map:
FPS: 1275 FrameTime: 0.784 ms
[ideas] speed=duration: FPS: 4038 FrameTime: 0.248 ms
[jellyfish] <default>: FPS: 7342 FrameTime: 0.136 ms
[terrain] <default>: FPS: 790 FrameTime: 1.266 ms
[shadow] <default>: FPS: 6002 FrameTime: 0.167 ms
[refract] <default>: FPS: 4273 FrameTime: 0.234 ms
[conditionals] fragment-steps=0:vertex-steps=0: FPS: 9208 FrameTime: 0.109 ms
[conditionals] fragment-steps=5:vertex-steps=0: FPS: 8964 FrameTime: 0.112 ms
[conditionals] fragment-steps=0:vertex-steps=5: FPS: 8984 FrameTime: 0.111 ms
[function] fragment-complexity=low:fragment-steps=5: FPS: 9360 FrameTime: 0.107
ms
[function] fragment-complexity=medium:fragment-steps=5: FPS: 9214 FrameTime:
0.109 ms
[loop] fragment-loop=false:fragment-steps=5:vertex-steps=5: FPS: 8945
FrameTime: 0.112 ms
[loop] fragment-steps=5:fragment-uniform=false:vertex-steps=5: FPS: 9218
FrameTime: 0.108 ms
[loop] fragment-steps=5:fragment-uniform=true:vertex-steps=5: FPS: 9077
FrameTime: 0.110 ms
=======================================================
                                  glmark2 Score: 7197 
=======================================================