I don't know where this nonsense about acceleration performance comes from.
Radeon has a memory bandwidth of 1.2 GB/sec. SM501 has half that. Both have similar command overheads and both are behind the podule bus. So unless the ViewFinder driver of RISC OS 6 is absolutely shoddy compared to the VPod's (and even not then, since for operations where acceleration is most relevant, overhead is least so), ViewFinder will generally be much faster than VPod.