I've used the Magnaflow you linked in your post on some vehicles, never a diesel though. It doesn't do too much to attenuate sound in my experience. Also, the cat does a lot to kill the diesel smell and some noise. I'm in no way a "green weenie" or a tree hugger, but in the case of a Roxor the benefits of the cat outweigh the single downfall, which is after some time it may begin to clog.
A full 2.5 inch system will be loud, but how loud is too much is a matter of individual preference. My cat back 2.25" system has a flow through muffler and it's perfect for me, great sound without being too loud. Under load in some situations it barks pretty good, wouldn't want it much louder. And one other thing, you'll really like side exit better than rear. Rear exit will swirl the fumes right back up to the passenger area and you'll eat fumes while driving. Side exit avoids 90% of that issue and VERY rarely you'll catch a whiff of exhaust