I have to confess that I don't know any Tuner who offers a 3.8 or 3.9 kit for the 996 Turbo. I know several Tuners who do that on the GT3 engine but not on the 996 Turbo engine. I asked a bit around and main reason why Tuners don't modify the TT engine to 3.8 or 3.9 is ... cost. It doesn't make sense on the TT because Tuners usually can obtain up to more than 600 HP without having to play around with the displacement. And the kits over 600 HP are usually not approved by german authoroties (TÜV/DEKRA).

I can't exclude however that one or two Tuners are doing the displacement thing on the TT engine but probably not for the german market. Sorry.