help with exhaust problem on gilera stalker

hello i have a friend with a gilera stalker it is total standard so he went and got a tecnigas exhaust and put it on now it feels like the revs arnt as high as before with the standard pipe on why is this can anyone help. thanks

 

Ive said this a million times but il say it again! You cant just bolt on an aftermarket exhaust and expect it to magically work!

You need lighter roller weights, thats why its slow & boggy off the line. Stiffer clutch springs would help also.
And upjet the carb by approx 5%.