I don't have the hard facts on this, but a while back, there was a hybrid turbo K04 being installed on a few 2.0T's over on
www.mk5golfgti.co.uk.
Assuming this is the same hybrid turbo, as I recall, the turbo itself needed to be in near perfect condition for the hybridisation to be performed, but it would not be known what condition the turbo was in until it was cracked open. Pretty sure the price to hybridise the OEM turbo was around £1k plus removal and refitting. In addition, uprated injectors (RS4 ones I think) were needed as well as a different map.
Add the labour for the injector/turbo fitment then the extra cash for the map together and you are getting to approx £2k assuming you have no turbo issues. And that assumes you have all the supporting hardware like exhaust, intercooler, intake, fuel pump.
There is no way an OEM K04 would flow enough air to make 420bhp, but even if it did, head gaskets failures, piston ring issues and the usual valve issues etc have been known on several highly tuned K04 cars. A hybrid turbo, without uprated gaskets, rods and pistons etc running a severe map would probably result in reasonable bills sooner rather than later.