By "400mph ugliness boundary" I mean the violient shaking and near compression (unless you are at a low alt.) I've gotten it to 500 by WEP diving it from 30,000 feet and leveling just above the trees (1500 where I tried it) and I had to fly it off of trim only until it dropped below about 475.
Experienced P-38 drivers in this game can reach speeds up to 500mph and maneuver without resorting to using trim. Sure, controls are rather sluggish at those speeds but you can still maneuver.
I suppose if you started at 35-40K and dove to sea level one could see over 500, but you were surely using trim as a primary means of survival b/c your controls would be useless.
You don't need to dive from 30-40k to reach 500mph+ in the P-38. You can hit that speed from dives at 10,000ft, just unload to accelerate at the beginning of the dive and you'll quickly hit 500mph and again, don't need to use trim to recover.
There are quite a few of us that routinely get our P-38s up to those speeds and never have a worry about compressability. YMMV.
ack-ack