Recently fitted a sip pipe to my standard t5 ,runs ok 1st to 3rd with lovely powerband but its absolute shite in 4th when presented with any kind of incline or two up,all ive done is upjet to 112 main,hows everyone elses run?any top tips?plus any info on timing set up for pollini kit,will it run ok with normal head or do you have to get it profiled like mallossi?  
You’ll need a T5 fourth gear if you want to pull on hills or two up. Even with a tuned Malossi kit and 30mm carb, mine still struggles in top gear as it can’t keep up the revs. The gear fits straight on as it only has one less tooth than the 200 4th. This may be the cheaper option than buying a kit and you can always fit a kit later if you don’t notice much improvement.
I’m not too sure about the Polini kit, but the malossi needs reprofiling to reduce the compresion ratio to prevent pinking. You’re best ringing a supplier as they will know more about it.
Malossi 210 ported by Taffspeed, SIP Performance exhaust, Malossi reed block and 30mm Dellorto. Taffspeed said it was a very big jet, but the engine’s very thirsty. Bugger to set up as well.
A 145 main jet, thats massive, what setup do you have?
Just realised what I’ve said! I must be asleep as I thought you had a PX200. I do have another sugestion though. My brother fitted a PM pipe to his standard T5 and had the same problem, but this was cured by a Malossi 172. I think you may as well spend the little extra on the Malossi kit as it will be more reliable and has the power to pull fourth. His engine was set up by PM Tuning and reached about 16hp at the back wheel with a standard carb. The only other modification was a 119 main jet, but you may need to go bigger for the SIP pipe as on my 210 is running a massive 145. As you said before you do need to get the head modified to fit, but once you have the 172, I doubt you’ll want to go back to standard.