Breathing: I read about someone replacing the stock TDI snorkel with a VR6 snorkel to get a wider diameter intake. Does any one have info on this idea?
Don't worry about the air intake if economy is what you're after. Even for performance mods, you have to do a lot of work before the intake needs upgrading. However, the intake manifold and egr may be clogged and need cleaning. This is common.
The power loss you're talking about could possibly be a faulty MAF sensor. It's mounted just downwind of the airbox and its job is to sense how much air is going into the engine and adjust fueling accordingly. Try unplugging the connector and give it a drive. If the power is improved then a new MAF is in order, cost $80 or so from
tdiparts or
impex.
Exhaust can be left stock also as not really any point in playing with it for horsepower. Search for "mufflerectomy" for ideas about removing the muffler for a sporty exhaust note.
Your stock clutch is generally fine for normal (non modded) use but will need upgrading if you go down the chip/injector road.
Tires and wheels? don't know really but expect a hit on mileage if you go bigger. That's what i've read here.
For more in depth knowledge about performace gains, try a search or a post in the performance section. It all goes on over there.
power enhancements