There are no plans for a faster version. This one is already tweaked to the max, and the design is based on a QFP CPU, which is not available as a 50MHz version.
Compared to the Blizzard 1230-IV, which uses a 50MHz processor, the ACA1231 is still faster, despite it's lower clock rate. While the Blizzard 1230-IV shows 9.88 MIPS in Sysinfo, the ACA1231 shows 9.94MIPS, sometimes 9.98MIPS (Sysinfo isn't all that accurate and depends on cache alignment and setpatch version).
Don't let the MHz number confuse you. The ACA1231 is absolutely in the highest 68030 class that you can think of, and the reason is it's superfast memory. The first-access penalty is even faster than on the old 56MHz card, which makes it faster on a cache-miss.
If you give me a few testcases, I'm happy to run them for you. I think I've posted an AIBB module before, didn't I?
Edit: AIBB module is in post#2211: