My "other car" is an '08 Speed 3 bought new in '08 and maintained and modded by me ever since. I am tuned on a K04 and running a 3" catless dp/rp into the stock MS3 CBE, which I recall is about 2.60". The turbo benefits from the 3" free flowing dp/rp, but in the Mazdaspeed community we found no real power gains on the K04 by increasing the size of the CBE. By the time the exhaust gas gets that far back it has cooled and compressed enough to flow freely through the stock CBE.
Not suggesting you shouldn't try going larger, just don't expect any real gains on pump gas.
The K04 flows much better than the K03, but is subject to center shaft failure if exhaust gas temps get above 1500 degrees F. That limits boost to about 15 psi from and above 6,000 rpm. You can go as high as 20-22 psi on pump gas down low if you gradually taper beginning at 4,000-4,500.
We had a lot of K04 failures on the Mazda MZR engine due to center shaft seal failure. That was not a fault of the K04, but rather due to excess crankshaft pressure caused by a design flaw in Mazda crankcase ventilation system. You will not have that problem adapting it to the 2.0T Audi engine. But, a used K04 from a Speed 3 may have damaged seals. These "smoking turbos" can be rebuilt, so you might want to get a kit and take care of it on the front end. It did us little good to rebuild, as the high crankcase pressure would just cause the rebuilt turbo to start smoking.
We resorted to aftermarket fixes, some working, some not. One solution, which has worked for me is a hybrid turbo using Garrett GT28 series CHRA modified to fit in the K04 snail. Again, you'll not have that problem if you replace the seals.
BTW, I have a K0422-882 that I will give someone for free if you pay for shipping. It smokes and will need seals, but is otherwise in very good shape. It was replaced by a BNR stage 1, which is the K04/Garret GT28 hybrid with much more stout center shaft journal bearings and seals, but identical flow.
The K04 is good to about 310-320 whp on pump gas, after which it is outside its compressor map efficiency and just turns into a hot air flame thrower.
It has very rapid spool, incredible low end grunt and nice power to about 6,200 rpm if you taper boost, keep AFRs relatively rich and crank up timing. Make the power up top with modest boost and aggressive timing.
I will follow your thread. Keep us informed of the progress.
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