That's because you, and all of swatt in fact, have probably NEVER seen a good gunsmith lol
You can get a stock marui P90 shooting 200ft with nothing more than a $5 hop rubber.
The theory on how the gun works is no different than an M4, only major differences are a slightly better mechbox design and a different hop chamber.
The only thing really holding the P90 back is the hop chamber, but I don't know enough about it to really pass judgement.
Now I don't know if it's just coincidence or demographic fact, but people who have P90's tend to use them mostly indoors, and just never seem to put any REAL effort into upgrading them....
Honestly I can't say I've EVER seen (in person) a P90 with more than $120 of upgrades in it.
Works the same as any other AEG, so I bet I could make one shoot 260ft and get some damn good accuracy out of it.
Heck, if someone wants to ship me their P90 I'll do the labor and testing for free just to prove a point!