its always an option. But I've seen dyno of the GT35r in T3 and the T4 and they performed identically. Granted both setups weren't the same but the T4 flange still necked down to the same 68mm turbine and the openings on the inside of the housing where it meets the turbine were exactly the same as well.
The T4 flange prob flows 2lb more than the T3. Negligible in my application since I'm not pushing the turbo to over 550whp. Even so, the T4 is just a lot more packaging headaches for minuscule gains. Its like going from a .82 to a .86 housing. Pfft, pointless.
Here are some views on the T4 vs T3 debate I found in google:
T4 vs T3 Spool and HP Difference??? - evolutionm.net
A lot of ppl say T4 is the ONLY way to go, but I made really great power on my T3 TS GT30R and the response is stupid.