Not enough to make a difference. The point of a trainer is to teach the pilots to fly the fighter, so it has to fly as close to the way the fighter
does as possible, or it doesn't make sense to have a two seater of the fighter.
I believe that in the Typhoons case the "twin sticker" is fully mission capable with perhaps a marginal drop in performance and range due to the
extra weight of the Navigator, ejection seat, canopy etc.
A really good site for info on the Typhoon can be found at www.Eurofighter.com
Most of the two seaters are fully operational versions of the single seater, with the added pilot being able to help out in a dogfight, and keep an
eye on any bandits they're up against.
slightly off topic but the other day down here (also in UK) in Gloucestershire the whole town turned out to see a spitfire and it pulled an awesome
and totally vertical climb! outstanding!
Naturally, okay, so a two seater got bounced and got em back, i concede that the aircraft is largely the same, but it must still speak well of the
aircraft as RAF pilots havent been flying theyre birds nearly as long as F-15 jocks.