You can do anything you want it is your bike.
However I do not recomend cutting up or drilling stock pipes.
reason:
1. They are expensive if you need to replace them
And there are many reason you may want to retun the bike to stock pipes at some time down the road.
2. You need back presure for the bike to run right and it's best
Good slipons are designed for the job.
3. Slipons you know what level of sound you will get.
4. slipons are not that expensive when you comsider you get to keep your
stock pipes
I sure others will pop in with thier thoughts