Im the worse person to ask bruva, cause Im a meathead when it comes to big bore engines. The bigger the better as far as Im concerned. Big turbo, Big sticks, Big exhaust, Big HP. lol.

The 38R is a wicked turbo. Uber quick spool (that sucker will hit 35psi in blink). No smoke and a nasty sound. And for the price Im thinking of getting a heap sent over and marking the hell up on em (like they all do here) and flogging em off.
The 4" is quite do able, dont get put off by the nae sayers. I cant quote you any real figures but just have a read through the forums. All the USA bruva's run with it and thats not just by co-incidence. The bigger the exhaust the quicker its gunna get it out. Now the cheapest I could find out here was $1600 and that was with a 3.5" downpipe. Clay has the stainless steel for $500 but if I was to buy one again I would just go for the ally one. With the shinny tip
Riffraff Diesel: MBRP 4" Turbo Back Aluminized. We dont have the rust issue that the yanks do with all that salt they throw on their roads.
Another say $250 for shipping then take it to any respectable exhaust shop and get em to fit ( no port a power here m8). Its cost me $400 for fitting. Just over $1000 and you are done. If anything you will luv the note.
I would have to say that the 38R will reduce your fuel economy. Cause all Im doing now is flogging it so I can watch that boost needle jump up. Lol. Seriously I havent noticed any difference and I log my mileage every fill up.
Thats my take on it m8