Do you uber 38ers manual trim only? I'm trying to get around the damned compression problems with the things in a dive. 
Yes, I only use manual trim. As for staving off compressability in a dive, 1) You will only enter into compressability if you start your dive above 20,000ft, 2) use your throttle and rudders to help control your speed in a dive.
Honestly, if you enter into a compressability state in a P-38, you screwed up. It is very easy to avoid compressability, if you know how.
ack-ack