Hi nice T-Jet I like the rap yellow, personal perception plays a part in that colour on the forums. Anyway it depends if your passionate about tuning your T-Jet if yes then its probably worth your money, some modifications are more obvious to start with than others like induction and exhaust, then moving onto mods like the Abarth turbo and forge intercoolers. If your worried about your warranty the best thing to do is speak with your dealer and ask if certain modifications will effect warranty, they may just void the warranty if the problem is to do with the mod itself or something its caused, modify at your own risk

its all good though mind
To begin I'd start with a GSR or Forge induction kit, both have proven good across the forums, GSR being the more prominent as its been around longer and for other reasons ill explain.
Secondly I would opt out for the exhaust together with the induction giving your T-Jet optimum intake, letting the car breath will have some gains in BHP and throttle response. You say Supersprint, they are a good exhaust manufacture I shouldnt see any problems there. Im sure someone will give advice on exhaust manufacturers and their exhausts.
Next Id say either the Abarth turbo or the forge intercooler, again the intercooler will maximise the T-Jet ability to breath easier giving more gains in BHP and working with the new turbo to take strain off the turbo and let it work "better"

The Abarth turbo is a good upgrade as its apparently a straight swap for the existing T-Jet turbo, with bigger internals the Abarth turbo is considered a good upgrade and again with better gains in BHP. If purchasing an Abarth turbo I would opt for a re-map not long after as the turbo works a lot better from knowledge if the GP has been mapped
Lastly the Re-Map, not being biased but your probably best going to red.dot for a re-map as they are considered the better guys for the job when it comes to mapping a GP, now earlier i mentioned the GSR induction, red.dot offer a remap and GSR induction together with a RR to show the gains together, this maybe an option for you.
Now prices:
GSR induction - £170 approx
Supersprint exhaust - Not sure on price id say around the £250 - £300 mark
Forge intercooler - £330 approx
Abarth turbo - can vary, normally around £100 in classifieds , Fiat as £400 for reconditioned, hope someone can shed more light on this for you.
Re-Map - £300 around that figure. Although red.dot on a RR day can do for £200 IIRC
Theres your guide my friend hope this helps, and im sure many of the well knowledgeable forum members will point you in the right direction some more.

Sorry if it looks like a book in the thread
Matt