People are *already* paid to work on the radeon drivers. Though I'm sure more effort would not go amiss, how many Linux X driver or DRM hackers are there who know how the radeon hardware works in sufficient detail, aren't already paid to work on it, and haven't got enough spare time to work on it? Plus, radeon support is fairly complete now, excepting only GPU computation and accelerated video decoding support.
More necessary I think is work on Mesa itself: it routinely comes out of benchmarks half an order of magnitude or so slower than the proprietary competition, which is unfortunate. (But maybe Gallium fixes this? I don't know. Also half those benchmarks are Phoronix benchmarks so take them with an entire ocean-full of salt.)