here's where i chime in!
ok, here goes:
first things first:
c2=gold. get the MOST AGGRESSIVE file they have.
you'll need bigger injectors for sure, as the aph has smaller injectors (270cc-ish as opposed to 318cc in MOST other models). also, don't forget the 4bar regulator as well!
a k04 is a bolt-in affair, you can re-use your stock (garbage) inlet pipe, but why? get a samco boost hose kit. by the time you do the k04 swap, you could have done a decent t3/t4 setup, so there is a price to pay for 100% compatibility.
for exhaust, 3" is a must. wait on the downpipe until you know what turbo you'll be running. k04 can use oem, others can't. skip borla as it sounds like horsescheisse and get the ghl 3" for a gti. yes, i said gti. yes, it will fit.
aph is narrowband, but not impossible to tune. c2 can make a file for it that'll blow the doors off. not worth the time to convert to wideband, you need that $$ to ditch the injectors and other crap they used to neuter the aph.
the aph has a smaller intercooler also, good call on the tyrol. i personally like front-mounts though, especially with a nitrous fogger on them.
if someone's interested, i have an aph stg3 turbo kit, brandy new in box with a 99 ecu.