I've fired up a few motors open. They tend to sound very healthy, and you can't hear the smaller stuff.
I'd splurge on a couple feet of flex pipe (if needed) and a pair of mufflers.
Guess John and I agree on this one.
If you want the roar, toss the mufflers in the back for the drive over to the muffler shop.
It's important to use all senses on a new motor...