My own ACA1231 gets really hot as well (can't bare touching it after some minutes).
I haven't experienced any Gurus yet, but some instability was noticeable after some time. I was about to go buy some low profile heatsink when I saw Jens comment about Vesalia shipping the heatsink kit. Nice gesture tbh.
So far, ACA1231 is running pretty great kicking an 1MB custom kickstart of mine and with "cache" and "burst" options on.
If I add any other option that works on my ACA630 (like fastchip, fastz2 or z2cache) makes the ACATune not kicking the ROM and not using the cache/burst options.
Without using ACATune I get ~4.5MB/s transfer speed on my 1GB Sandisk CF, while using cache/burst on I get ~5.4MB/s (both speeds using Idefix Express without software installed).
Will have to give more test with different CF's (I have 4GB Transcend 133x and 8GB Sandisk Extreme) to see how it's going or else I'd have to send Vesalia my adapter for a boot-delay firmware change.