so I bought an exhaust tip because the stock 1.5" or so outlet looks stupid with a huge 4" + cutout on the bumper

it is a resonator tip but the tubing from the stock goes past the holes

I was wondering what would happen if I cut the exhaust to go into the holes?

 

Even with the GM optional exhaust tips, you have to shorten the tailpipe stub that comes out of the resonator. If you lop off the excess it will look better, whether the resonator tips actually do anything, I can't tell you....they seems a bit like snake oil to me.

But you do have a point, the bone stock tailpipe on the HHR is a sad looking little thing unless you use black high temp paint to make it and the resonator behind the bumper "disappear".