I have done some tests on that ealier this year (in our print issue 02/2010, fwiw) with the transcoder also used by third party products like Power Director 8, which i used.
All Radeon cards were strongly accelerated using the „GPU transcoder”, but from what I've measured, the difference between a rather lowly 4670 to the mighty 5870 was 2 secs (63 seconds to 61 seconds total) for our test clip converted to the "Iphone best" preset. When encoding to H.264 AVCHD, the difference itself became larger, but also more variable. HD 5870 was 20 seconds faster than 4670 in a total time of 191 seconds vs. 211 seconds. I attribute much of the difference to core clock, because a 4830, which should be fast than 4670 in general, scored worse both times. 5770 and 5870 were identical.
So in conclusion, I'd say, there was either no or only an insignificant part of the work done by the shaders. The Geforce models from GT220 to GTS 285 (no fermi back then) scaled much better, but also more with clock rates than shadercount.
Also: The image quality produced was slightly different despite using the same presets - CPU output was best. Small differences in „optimizations” would help a great deal here.
All Radeon cards were strongly accelerated using the „GPU transcoder”, but from what I've measured, the difference between a rather lowly 4670 to the mighty 5870 was 2 secs (63 seconds to 61 seconds total) for our test clip converted to the "Iphone best" preset. When encoding to H.264 AVCHD, the difference itself became larger, but also more variable. HD 5870 was 20 seconds faster than 4670 in a total time of 191 seconds vs. 211 seconds. I attribute much of the difference to core clock, because a 4830, which should be fast than 4670 in general, scored worse both times. 5770 and 5870 were identical.
So in conclusion, I'd say, there was either no or only an insignificant part of the work done by the shaders. The Geforce models from GT220 to GTS 285 (no fermi back then) scaled much better, but also more with clock rates than shadercount.
Also: The image quality produced was slightly different despite using the same presets - CPU output was best. Small differences in „optimizations” would help a great deal here.