K04-015 is pointless, its basically the same thing as a K03-073 they fitted to the 190hp B6 A4.
For "300" you have essentially three options as i see it:
BBT K280 or K300 hybrid. Bolt on, but probably wont actually make 300, if your happy to settle for 270-280 then great. You'll still need a high flow exhaust manifold and TIP to get these figures though.
GTRS eliminator like you suggested, will do 300 with the right bolt ons, you'll again need big tip and big manifold though. It wont do anything like 400. Even a full frame proper GT2860RS turbo will only do 350-360ish on reliable UK dynos.
TFSI K04-064. Capable of >350, but running a sensible 300-320 seems perfect to maximise longevity etc. Usefully comes with its own manifold which flows well, but it does require adaptor plate and ofcourse the downpipe and intake stuff needs modified to suit.
With all of these options you run the risk of engine failure due to the torque output. If you map it sensibly and keep the torque down, it will probably survive, but you need to be aware and accept that you might well be unlucky. If you map it maxed out, full boost, all the torque 100% banzai mode, then ofcourse your hugely increasing the risk.
Fitting a set of rods mitigates these issues, and a rodded stock motor will do 400hp without too much trouble.
Personally, if i were starting again doing an A4 build from scratch, it would be a K280 turbo on a stock internals motor, with the exhaust/intercooler/intake etc sorted out and mapped sensibly for 260-270hp, or a bit more if it will do it without wringing its neck.
If i wanted ">300" then i'd do a TFSI-064 on a stock motor with rods.