Even with the hardware UVD support on PowerPC under Linux there appears to be a problems with 1080p decoding . At least on the X5000/020 . I was not seeing any better performance with a Radeon HD6570 in the pciex16 slot as the FireMV 2260 in a pciex1 slot. Issue could be in hardware or software.
Under AmigOS 4.1 the higher end cards give a boost with video playback with the composite comp_yuv2 output. Also the Polaris cards have a much better composite performance for larger rectangle sized manipulations(Have not tested video playback on the X5000/020). A couple of 5% increases in performance such as the Radeon HD 3.x driver and a video card that has a better composite performance makes a big enough difference to be able to play 720P video well without altivec on the X5000/20.
Edit: I am also getting decent performance with 1080p MP4 videos .