it's not that simple, even the small turbo has to flow properly and efficiently for your power goals especially on the exhaust side, you can't just put on a k03 with a t4 and make 500hp. hy35 would be better than the t04e, although they are about the same size.
to preform perfectly no, but to preform well enough sure...
as far as exhaust restrictions are concerned, it will probably have a 38mm gate between the high and low pressure turbo's turbines. if that isnt enough flow i'll go to a 44mm gate.
the compressor from a 42mm t3 to a ~50mm t4 frame would work fine. if anything there may not be enough of a split and id be leaving some available boost on the table if i had a smaller high pressure unit.
i think that the 2 turbos you picked will work fine, but the point i am making is that if the exhaust side is too small on the small turbo you can't just put a bigger wastegate on, that won't solve the problem that it takes 50psi of drive pressure to make 15psi on the compressor side, thats an efficiency problem. but i don't think you will have that issue, i am just saying make sure that everything is sized appropriately, and turbine housing size is just as important. also are you running a straight t4 or a t3/t4?
Don't let the 11mm block scare you. Mine is tougher than nails with the Arp's.
Tough as nails? Ha ha. The ARPs will definitely reduce the odds of an 11mm block cracking... down from 90% to 65%... Getting a 12mm block reduces it down to less than 1% and will typically cost less than the ARP studs.
I have nothing against running a working 11mm block, but IMO it is not even worth replacing the headgasket on one.
yep once you have to lift the head off, just get a new engine. a tdi or aaz preferably haha