ok, some results:

PLL-readings on good working compilations:

3.15.0-rc1-00303-g6e66d5d
[drm:radeon_compute_pll_avivo] 69300 - 6949, pll dividers - fb: 165.0 ref: 2,
post 17

3.15.0-rc2-00042-g4d0fa8a
[drm:radeon_compute_pll_avivo] 69300 - 6930, pll dividers - fb: 329.1 ref: 4,
post 17

3.15.0-rc2-00077-g76e7745
[drm:radeon_compute_pll_avivo] 69300 - 6930, pll dividers - fb: 329.1 ref: 4,
post 17

3.15.0-rc1-00070-g2431581
no output, system hangs loading driver in debug mode
(probably because this one didn't had the patch yet.)
works ok when not in debug mode.

PLL-readings on bad noisy-artefacty compilations:

3.15.0-rc2-00069-g1aae31c
[drm:radeon_compute_pll_avivo] 69300 - 69290, pll dividers - fb: 135.5 ref: 2,
post 14

3.15.0-rc1-00071-gc2fb309
[drm:radeon_compute_pll_avivo] 69300 - 69290, pll dividers - fb: 135.5 ref: 2,
post 14

3.15.0-rc1-00075-g3ed9a33
[drm:radeon_compute_pll_avivo] 69300 - 69290, pll dividers - fb: 135.5 ref: 2,
post 14

3.15.0-rc2-00092-g92891ed
[drm:radeon_compute_pll_avivo] 69300 - 69290, pll dividers - fb: 135.5 ref: 2,
post 14

Problem is: I haven't the slightest clue what it all means.