I have to disagree with this - if you have a blown Turbo the K04-15 is great as long as it is does not cost too much more - I really think you are making a mistake if you go for the K04 when looking for a larger turbo conversion.
There is hardly any difference in size between the K04 and K03 and you are spending a fair wedge (eg MTM charge about £4k for this conversion, about the same as everyone else) - for this you get a whopping 20bhp over the K03 plus exhaust and cat conversion.
Having had a K03, then exhaust then K04 then GT28RS I would strongly advise against the K04, certainly on my car (1999 Quattro) the K04 was dissapointing to say the least - less kick than the K03 and only marginally faster in real world terms.
If you check out the following site
'ATPTurbo' you will see what I mean, look at the size of the intake on the K04 (same size on the K03) vs the GT28RS there is simply no competition and, from the install on my car (which is not an eliminator) the spool up time is pretty close, you just keep going with the GT28RS up to 300+bhp.
Basically - having been through the loop I rue the day I bothered fitting a K04 - unless one falls into your lap it is, in my opinion, a waste of money.
Don't take this as a slag off of anyone who has a K04, after all I went down that route myself I just cannot recommend the outlay for what is a minimal perceived benefit; presented with an already blown K03 the picture may be different.
Hope this helps someone, somewhere and look forward to other opinions...
Cheers,
Greg