I did some test flights,it seems now you don't need to worry about compressibility ever if you fly below 7500m,really you could dive straight to the ground and never experience serious compressibility.
Starting from 10000m though,it does exhibit compressibility at around 8km,but even then when you get close to 4.5km,you can again control your plane.
Either
1.the compressibility effect is under-modelled,actually by using
http://www.globalaircraft.org/converter.htm and compare against in game speed using open cockpit TAS gauge,i think it only get into uncontrollable dive above 0.8 mach.
2.or speed of sound is overestimated,yeah high precision mach number vs altitude/temperature is the thing that get classfied,and i'm totally a layman on this,but it can't be that far apart does it?
3.i just made a mistake in my assumption related to mach number because i'm a layman,but i tested in hot map(Coral Sea) and cold map(moscow),and it's all the same,below 4.5km you are immune,and not that pronounced above that.
4.or that the non-pronounced compressibility effect is modelled without considering every sim player can fly with joystick like they have hydraulic assists,and this game have not so accurate modelling of trims(actually much like a seperate low drag control surface,especially at low speeds,this solve the problem of uneven feels of moving main surface in opposite direction,exhibited if you simply model trim as main surface offset,but it can be exploited in certain situations where main surface should be too draggy to be usable.)
For trims,it looks like now trim is off by -3% compared to previous setting.
For 110% throttle Sea Level level flight with 50% fuel on Coral Sea Online map,
previously you need -23% trim,now you only need -20%,
For climbing at best climb rate speed,it was +15% trim previously,now it is +12% trim.
They have same max speed at sea level in 4.11 and 4.12.
It is a change for the better,now you don't get annoying pitch up when aiming using default trim,but i think this warrant a entry in change logs.