A slipping clutch is not going to make any difference to the way your shift feels on engagement. You'd smell it soon enough after a few tries.
Sounds more like fuel or air starvation or boost limiting to me with only the thin info available to go on. If you have no CEL's that makes it trickier to diagnose unless you go to a dealer or have a Dashhawk. I'd consider:
How many kms on the car, is the air-filter stock or has it been altered and if so, how, is it blocked (plastic bag sucked into the intake, etc). Possible fuel, pump feed issues or line blockage, etc. Is turbo wastegate sticking? These are just a few suggestions in 3 separate areas.
More specifics if possible, please.
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18