Skepticism is fine. But I don't think it's worth worrying about if the output claims are legit.
Firstly, sigh the ST community really needs to understand something. When you deviate from the reference product all sorts of unintended things could happen. For ST owners the G series will solve a couple of problems.
1) It narrows selection down. Even if more product is on the way and it is, it's for mid-frame turbos that most of you aren't interested in anyway. Both turbos are similar in size to the GTX2860 and GTX2867.
2) Mid frame power in a small size. My GTX3071 is rated at 650 hp. Where I cannot get clarification is if that is at the wheels or at the crank. If it's at the crank, 578.5 hp (about 11% driveline loss) which is fine but if it's really 650 then that changes the fuel system requirements quite a bit.
Also with the smaller TB-04B compressor housing I get the smaller 3" inlet (outlets are the same) and I also don't get the additional benefit of the ported compressor housing. The G series eliminates that problem in a vehicle specific kit like the ATP I would enjoy all the benefits of the reference turbo.
This is why I put max flow of the GTX3071 at 550 hp at the wheels. Powned has already hit 480/420 at 26psi in the early stages of his tune. It should make 500 at 27-28 psi and 550 or so at 30-32 psi.
With the G series it's simple under <500 = G2-550 500> = G2-660
Where I don't like the GTX2867 is that it's slightly mismatched. If you have to give up 200-250 rpm of spool to get 25-50 hp I don't think that's a fair trade-off. Where the GS-660 spools likely the same way, you get a whooping helping of output.
Max output (if the hp ratings are at the crank)
550 = 489.5
660 = 587.4
Full Race and ATP both have these in-stock.
I've been told that for 2.0L .72 A/R is ideal, .49 A/R is for under 2.0L think Fiesta ST.