Gunnar did explain on the Apollo forum how to make use of HP on AmigaOS for some real speedup.
I read his post and he suggested using one register set for user mode and one for interrupt mode, purely to avoid the movem.l overhead when processing interrupts & not actually run code using the two register sets simultaneously.
It should be possible to make movem.l into cached ram pretty instantaneous though. Which would benefit calling ever single function.