If it feels slugish then theres really 3 things to look at, ignition, fueling and intake
Ignition, probably most likely get miss-fires if there's a problem here
Fueling, probably get some hesitation under full load/at high rpm
Intake, probably see less power at high load/rpm
The other posibility is a blockage in the exhaust, possibly damaged CAT but I've only heard of this very very rarely
Cost of diagnosis is normally £30-£50 or if you know someone with a reader or scangauge they could help you out