I've ended up forking out for an ACA1232-25. However after a months or so of using it, I'm disappointed with the performance.
Sure, workbench generally runs MUCH faster, however most games/demos that make heavy use of the custom chipset actually run slower with the ACA1232 than with the ACA500 or even with no expansion at all.
This is clearly evident in a number of games and demos, whdload or otherwise. For example, Zool, James Pond, Spaceballs-Spasmolytic demo, all get slow downs, stutters and zool even gets video tearing. No amount of fiddling with whdload config or tooltypes has fixed it. If I make a floppy of the game/demo and boot off it I get the same slowdowns so I know its not related to whdload blitter patches. However, if I disable the ACA1232 using the disable-jumper, everything works fine.
I ran bustest and the WHDLOAD memory speed slave (in the dev package under /src) and I found that chip bandwidth actually seems to reduce when the ACA1232 is enabled. I can only assume that whatever the ACA500 is doing to make the A1200 accelerator work incurs some performance penalty when accessing the amiga chipset.
I'm finding more and more often that I'm disabling the ACA1232 to make things run properly. So I am thinking I will go back to my original plan to get some A500 memory expansions and sell the ACA1232.
So, I am again looking for a trapdoor exansion with ~1.8MB and a Gary adapter, or any other RAM expansion that works with the ACA500. I see fitzsteve has a 'GTI-edition 68k-fastram' 2MB expansion, that clips under the 68000 CPU on the motherboard but is confirmed working with the ACA500, so ideally I want something like that.