Ok i made some tests... ACA1231 is indeed fast.
Transcend 133x CF's don't seem to work (they give Kickstart screen without boot) while SanDisk (classic blue ones) work just fine.
I got some nice 5.2MB/s using cache/burst on on my Idefix Express (without software installed) and
about 4.2 without cache/burst.
Newest ACATune (1.4) doesn't work well in my S-S. Tried to map an 1MB ROM and I was always getting red screen and reboots.
Putted back the old version (1.2b) and problems disappeared.
Btw, except the classic Burst/Cache options, the others that work on older boards (z2cache etc) don't work on the new board yet?
Will give more tests tomorrow cause I was really tired to test it thoroughly.