Those ribbed sections are bad for flow, but I doubt that you'll notice it from a seat-of-the-pants perspective. It'd be interesting to see a dyno comparison of mandrel bent vs traditional, but it'd be difficult because the bends would have to be exactly the same, same muffler, etc etc. Most of the mandrel kits will come with higher performance parts.
As far as the muffler goes, go with any listed as "stock replacement" or even the budget ones if you want quiet. We sell and install Walker where I work, pretty decent stuff for replacement parts.
