It can't be that easy...

[psxphill]

Isn't this pretty much how the AdSpeed worked? For what it is/was, it was a pretty nice kick-in-the-pants if you couldn't afford or source a better accelerator.

Adspeed will also cache fast ram.
 
http://amiga.resource.cx/manual/AdSpeed.pdf

[psxphill]

Supraturbo 28 had tiny 16 kB sRAM cache to give boost when fast RAM was available.

I have one, it was awesome.
 
http://www.amiga-hardware.com/download_photos/supra28_6_big.jpg