Hmmmm, could end up with a lot of contradicting info on this thread.
For what it's worth here's my 2p:
What budget have you got? How mechaninally minded are you? Do you want to be able to return the car to it's original condition at any stage?
If you don't mind spending a bit then here's what i'd do:
K03s Turbo (£250)
Milltek 2.5" Downpipe and Race CAT (£400)
Green cotton panel filter and 80mm Air Feed (£50)
Forge Motorsport Turbo Intake Pipe (£120)
FMIC (anything from £250 to £750 depending on how good you are with a spanner!)
Depending on how good with a spanner you are, you'll need to add garage labour to some/all of those prices.
Once all those parts are fitted you'll need to get a remap. Something like REVO won't cut the mustard as it isn't designed to work with the K03s on an A3. You'll need to go somewhere like Jabba; they won't give you a custom remap (not matter what anyone tells you), it'll be a Dimsport generic map but what they will do is alter the fueling and timing to match the increased boost from the K03s. Think of it like having the sleeves and trousers lengthened on an off the peg suit, rather going having a bespoke one tailored from scratch
. People call that 'custom' but it isn't, the benefit is they'll do it in a couple of hours, it'll only cost £600 and your car will fly.
If you did all that i'd say you'd be looking at a genuine 210 - 220bhp & 250lb/ft at the flywheel.
Problem now is you'll have crappy brakes and lame suspension...