Two things....
The under coating won't hurt, and I guess every little bit counts, but don't expect your Honda to turn into a BMW overnight. For the best surface finish, you can't beat 3M Undercoating. The only bad thing is that it's $7 a can. For this application, buy the cheapest thing you can, and add a few coats. I would tape off anything that you don't want coated in rubbery goo.
Second...don't buy dynamat extreme. It's a waste of money. There are other alternatives on the net that are cheaper and just as effective if not better than Dynamat. One of the best alternatives is Raam mat. Its sold by Raam on ebay.