Hi guys i need some advice

I drive a mk4 gti and recently the car has not been behaving.For some reason the car seems to bog down on the low revs and it takes the car forever to reach 3000rpm.Everytime i accelerate the car seems to drop more in revs.the car at the moment has a full exhaust system and a cold air intake.i also checked the faults and it was a o2 sensor that was throwing a fault,so i changed the o2 sensor but the car still struggles to pick up revs.its also worth mentioning that the car has done 270000km.is my motor on its way out??

I need some advice on what to actually do or who i can take my car too,to get it checked out.

Have you checked boost pressure and the turbo wastegate actuator? Any whistling sounds? Air leaks?

Fuel pressure checked?