All of the downpipes are going to be pretty much the same. The biggest difference that you will find will be in the build quality/fit and finish and the quality of the cat's. Something like an HJS cat or any other premium cat is probably going to perform a little better and have a little less risk of collapsing and causing restriction than the cat in one of the USP dp's. Having a cat collapse is a rare occurence though. Not saying that USP dp's or cat's are bad either. Plenty of people on here have used them without issue. Best reasons that I can come up with for using a higher quality cat would be higher flow rate, less risk of failure and better catalyzation. I'm not saying that any one type of dp/cat is any more or less failure prone than any other, just that in general you get what you pay for.
One big thing to look at when considering a catted dp is the cat's cell count. Something with a 200 cell cat will flow better than a 400 cell count.
Alternatively, if your state doesn't do emissions checks you can just go catless.
Some good dp options to look at would be Bull-X, Billy Boat, APR, CTS, USP, Eurojet, Unitronic, Revo, AWE, Milltek, VWR, and Invidia.
Some of them (Bull-X, CTS) have 3.5 inch turbo outlet sections with 3 inch piping the rest of the way. Others are 3 inch from the turbo back. So in theory, the ones with the larger diameter piping will flow better.
Another thing to consider is the thickness of the walls of the piping. From what I've seen, the Bull-X one uses thicker piping than most of them. I have one of the Bull-X catless dp's and can definitely say that the build quality is excellent.
Here is a photo of what the Bull-X catless dp looks like next to the stock one:
http://imgur.com/EuZF85H
I don't have a closeup of the welds on the Bull-X one, but they are fantasic. The beads are so nice and even and clean looking.
You can see the slight taper from the 3.5 inch section to the 3 inch section if you look closely.
Hope you plan on going Stage 2 once you get a downpipe. Especially since you have Eurodyne. The difference from ED 1.5 to 2 is night and day. I went from 1.5HT 93 to 2 93 and I love it.
You'll probably want to look into an intercooler upgrade if you haven't already. The heatsoak when stock is bad enough, let alone when running a tune.