Yep...they could fly at 30K. But I think that would be pushing things for the poor bananas in 190s asked to intercept buffs in our scenarios. Keeping 'em 20-25K is also historically "realistic" but doesn't totally put them in a range where the most common interceptors in AHII scenarios have absolutely no performance left.
From reading some of the reports in that, alot of variables probably went into what was assigned as alt for each mission. Escort type and range, weather (as stated), distance, Situation at the time of LW capability/local, ack reports, etc. Even the numbers of bombers in the attack mission made a diffence depending on your station (high, medium or low echelon).
I still think it would do more harm than good if a "cross the board" alt cap was imposed for all FSO's. If one seems necessary for a particular event, then impose it on that event. There are times when targets can be deep into enemy territory and fighter escorts are really stressed on fuel to even make the route (much less dogfight). There have been a few times escorts used fuel fighting on the way in and had to RTB even before the return route of the buffs. On those runs, at a cap of 20-24k (give or take), you may be looking at 4 or more hits from converging units or multiple rearms through 5,6,or 7 sectors before getting feet wet.
Then the high alt becomes almost an aid to survive.