You've nailed the problem directly on the head. For hitting 60Hz in FuryGpu, I actually render at 640x360 and then pixel-double (well, pixel->quad) the output to the full 720p. Even with my GPU cores running at 400MHz and the texture units at 480MHz with fully fixed-function pipelines, it can still struggle to keep up at times.
I do not doubt that a shader core could be built, but I have reservations about the ability to run it fast enough or have as many of them as would be needed to get similar performance out of them. FuryGpu does its front-end (everything up through primitive assembly) in full fp32. Because that's just a simple fixed modelview-projection matrix transform it can be done relatively quickly, but having every single vertex/pixel able to run full fp32 shader instructions requires the ability to cover instruction latency with additional data sets - it gets complicated, fast!
I do not doubt that a shader core could be built, but I have reservations about the ability to run it fast enough or have as many of them as would be needed to get similar performance out of them. FuryGpu does its front-end (everything up through primitive assembly) in full fp32. Because that's just a simple fixed modelview-projection matrix transform it can be done relatively quickly, but having every single vertex/pixel able to run full fp32 shader instructions requires the ability to cover instruction latency with additional data sets - it gets complicated, fast!