Make sure the really cheap ebay ones actually have a 3" - minimum - inside diameter before you fit it. There is some far eastern crap being sold as 3", but is actually only 2.5" ID, which causes massive back pressure. Ask R Tech about it!
The turbo outlet is approx. 84mm ID, 3" is 76mm, so ideally the bigger the better to avoid a step in the flow. Relentless performance on the TFSI facebook group sell a 3.5" DP which gets around that problem, and looks well made.
Another option is to bash out the first cat in the OEM DP, but make sure you do it facing the floor to avoid clogging up the second cat with crumbs. I put mine in a B&D workbench facing down and hogged it out with a 32mm wood drill. Took all of 2 mins