AFAIK all the modders do their very best to model as close to real world performance as possible with the game-engine, which means a lot of research to tie down exactly what their accurate real world performance and handling qualities were, and to find this well represented when using the strict flying conditions of actual mission building. In other words performing comparative tests starting on the runway using FMB and then performing a proper full flight mission to do a series of tests in precisely the same way manufacturer test pilots performed them historically. This is simply the easiest true measure since a fairly good sim like Il2 (when operated at full realism settings) models so many variables such as atmospheric conditions you run into complex diversity trying to do it all with math and formula, or just jumping in with the QMB.
But that's just the measure. The actual real world technical specifications are a part of the game-engine modelling of flight data, weights and measures and real world performance guidelines really only have to be converted to metric units before being incorporated into a new slot aircraft, so the more comprehensive and accurate the real world documentation researched for the aircraft, the more accurate the modelling tends to be.
The greatest variable with accuracy I think would really come in on effective and dedicated research backing a project before it is released, for which the modders at AAA have excellent reputation and demonstrable concern, and is the most likely reason for less realistic mods which might be available elsewhere. Quite simply some enthusiasts are one-eyed, or entirely unrealistic arcade-gamers which you run into at all game mod sites (the mega death laser and super invulnerability mod for FPS/RPG games for example, a common theme).
So generally speaking the real world research, if good quality should tell you what the performance of most AAA new slot/new FM mods will be. If you follow the pilot manual of the real thing for operational guidelines, you should get best performance from the mod version, we do our very best to mod them that way
Questions about a specific new slot aircraft, like the D-11 or MkI Spit ought to be directed at the mod creator via PM or in the associated discussion thread. I'm sure you'll find they'll happily discuss its correct operational guidelines and performance specifications with you. Some of us will talk your ears off if you let us
