None here. Downpipe and tuned 2020 on E30 made 471/553 dynojet. Factory unlocked ECU. 20 spools a stock turbine housing slightly quicker. HKS hipermax hybrid kit was designed for the 20 not the 21. Besides a 20 can run a 6 port head if you so desire and then have 11:1 compression.
Part numbers are different yes but due to manifold changes. The actual turbo wheel sizes have been verified to be the same. The 20 is tuned with more boost taper and less overall boost period. Remember this turbo falls out of efficiency at 22-23 psi so more boost becomes inefficient.
I'm trying to figure out what this hose is or does. It appears to be vacuum only. It is #2 in the diagram and connected from the intake pipe to the side of the block.
BMW has a valve that opens when under boost only to return crankcase pressure back to intake at least on the n55
In high boost where the smaller turbo tends to fall out of efficiency water/meth will help control the intake temps. I advise against running anything but 50/50 meth/water due to preignition issues. But have only ran 1 water/method setup.