Courtney, you are right to be worried about raw performance of the aircraft once stealth is eroded by time. But having seen the figures I would not use he term 'underpowered' as above and especially below the transonic region the jet accelerates like a scalded cat.
I readily accept though that the acceleration from 0.8 to 1.2+ is a real shocker for AIM-120 tactics and if you start kicking the aircraft around in the transonic region it just pisses energy.
Spending so long in burner at night in a supposedly LO platform will not be fun.
Just plain awful really.