I'll relate my own experiences of Golf ownership (see
http://www.oreton.com/cars.htm) and the other guys on
www.uk-mkivs.net ...
Between the K04-001 and k03s conversion, the K03s is the MUCH cheaper option and will produce about the same power/torque. As has been stated they are very similar physically although the general consensus is (and the vwvortex guys will testify to this) that the K03s is the stronger unit (and therefore more reliable in the long run) although my experiences show that the K04-001 breathes better at the top end 90mph onwards.
So you could bolt on a K03s (they can be had for around the £350 mark) and see 240bhp from a Jabba remap but thats your lot, maybe a few brake more with other mods like 3" DP, de-cat or sports cat, all the IC pipe work/Samco etc etc (again check out the mods on my golf). You dont need a 4bar FPR as Mike at Jabba 'modifies' /ubbthreads/images/graemlins/wink.gif the standard one.
You can push a K04-001 upgrade further with the hybrid versions like the E0-5 like Wilko off uk-mkivs.net did (he ultimately got fed up with that and went IHI) which should see around the 270bhp mark, then with FMIC, uprated fuel pump (S3/TT) and higher flow injectors you could remap up to near the 300bhp mark..
BUT BUT BUT, my point is here that you are really pushing the envelope of the fragile (using the term very loosely) K04. You will be pushing the boost up, perhaps running around the 20psi mark on WOT and to achieve 300bhp ish you will have spent close to what it would have cost to go IHI in the first place!!!
An IHI conversion runs lower boost for more power, yes there is slightly more lag and the power comes in later (approx 300rpm later than the K04) but at 2500rpm, even though the IHI wont be 'on boost' yet it will be producing more power than K04 at the same RPM!
The power delivery on an IHI is ultra smoothm there is no mad ramping up of boost, just a very linear power delivery.
An IHI will be mapped by Jabba at about 1bar (14 psi almost) and produce between 330 and 380bhp with MASSES of torque but they tend to 'limit' the torque to 300Ibs to keep the clutch sweet.
The IHI conversion is not a lot more work than a K04 conversion, its just the DP fabrication which is the pain in the ****.
All the parts can be sourced yourself and save yourself a good wedge. I think from memory the IHI is the VF-34 (probably way off there lol /ubbthreads/images/graemlins/smile.gif )model, the only thing you will need to achieve the 380 ish bhp is the custom made Jabba manifold (they dont make it but you cant buy it from the manufacturer). All the oil lines can be made by a local hydraulics co, the S3 fuel pump from VAG or 2nd hand, the injectors from many places and any mechanic worth his salt would be able to make the DP adaptor fit to your existing DP. Finally when its all fitted you take it to Jabba for the final mapping.
Btw, if you source the parts yourself, you can go to JBS or Dubsport who will do the fitting for you. I doubt Jabba would be very happy about being given the parts which they normally supply themselves, and being asked to fit them lol.
Jabba ran their MkIV on a standard SMIC for a long while with no problems, there is so much power and torque there that you hardly notice the heat soak /ubbthreads/images/graemlins/smile.gif
So in conclusion, with smart shopping around I would strongly recommend going the IHI route which will ultimately be more reliable and powerful if a little more expensive but at least then you wont need to upgrade any further.
Cheers.