I've had similar experiences. My old escort gti would not rev past 3.5k which turned out the cat had broken up and blocked the exhaust. Also had a mk4 fiesta 1.4 which revved up to the redline which seemed fine at the time but had no power, that turned out to be a broken up back box. Once I changed the back box it revved much quicker. Best way to check if your exhaust system is at fault is to put your hand up to the tailpipe and feel the pressure coming out the back. If there's little coming out then I'd suggest looking into a new exhaust.