The oem 8th gen si exhaust from Honda is oval like above. I'm guessing it's just an aftermarket axleback made for the 8th gen they are using in the first post.
You'll also want to cut more of your bumper or you're going to end up with a melted area. I've seen others have that issue.
Just bend the tangs a bit to get it lower. Also loosen the spring bolts and stuff a towel between the bumper and the tip. When you tighten the bolts again it should help to hold it in place so it stays aligned where you want it. Once it's tight again you should be able to pull the towel out and the muffler will stay. It takes a few tries to get the tangs bent just right and you need to be careful that you don't tear them off the muffler at the welds.
Just my 0.02 having done it a couple times.
Other than that nice score! That will look much better on there than the stock rusty pipe that stuck out. It should sound a little louder too!
EDIT: If you still need to cut you can always pick up a muffler guard. I think they are made for S2000 and EVOs. Carbing makes one I know of. AKA rear bumper exhaust heat shields.