If you go for midpipe only + cobb it won't matter much what pipe you take in my opinion. More a matter of taste/budget. I have a Miltek midpipe + Cobb with GTC custom tune and the difference is very dramatic. If I will upgrade to a full Ti system I will go for Akrapovich. It gives a little power gain vs most other systems although really not worth the money. So thats more about sound and taste as well. And being snobbish. As for service, both GTC and Miltek are absolutely great.
As Tom suggests the simplistic design of the Y-pipe means the choice of manufacturer will make next to no difference to the power output for a “stage1” (from whichever company) as there are other elements that will limit power.
The Milltek rear silencers have been independently (TUV testing) proven to flow over 37% more than the restrictive standard system but not all of that improvement will be realised without running more power than the standard turbos will generate.