It's one of the most common yet over-rated modifications there is lol, drilling a hole (or holes) in the stock can may provide marginal reduction in back pressure depending on which kit exhaust it is, but the alteration is so slight any change is mostly a placebo effect as the increase in noise makes you feel like it's more "powerful" - if it's one of these http://motorbicycling.com/f13/cataly...ers-15857.html
it won't do anything at all.
I played around with the kit pipe when I built my first ride, incrementally reducing the back pressure by trimming the interior baffle, drilling the exhaust tube & doing GPS runs... one cut at a time (2-smokers do need some
back pressure to run properly). While it did help to take about 1/4" off the interior tube - it "helped" by no more than 1/2 a mile an hour and even that may have been environmental (changing weather conditions), while the most noticeable effect was ofc an increase in noise.
An expansion chamber exhaust is in my opinion a critical
component of a two stroke left out of these kits only as a cost saving measure, it's by far the best performance effect you'll get from any modification or aftermarket part... here's some good readin' if yer interested;