TB993tt:

Cargraphic sell the 600PS/800NM kit developed by RS Tuning which a few forum guys have purchased, this will work within the limits of the standard intercoolers but even this will lose power when your ambient temps get over 30degC......

Your question specifically mentions instant response and reliability. Tuning the VTGs effectively seems to be a hard thing to do but the RS kit seems to be the best in this area, checkout the data on their VTG 997tt 624PS/826NM - it has amazing response and I can report from my RS Tuned engine that this is a trade mark feature of the RS engines - reliability is a given with this race engine builder.

Back to the Manthey kit - With his reputation it will be well resolved, I believe his VTG control and ECU programming   was done by Ansgar Weissen who has a good reputation and it is a viable alternative to the CG/RS kit......

The other thing worth mentioning is the torque delivery (and size of the torque ie sub ~850NM) - the smooth delivery of the RS Tuning really suits the GT2 and enables traction to be maintained - something which can be easily upset by a peaky tune - I would imagine the Manthey to be similar in this regard. 

personally I wouldn't reccomend any others for our expensive unique cars Smiley

 

Very true, both companies are involved in racing (Manthey won the Nürburgring 24h three times consecutively, RS built the engine for Alzen´s 997 Turbo). Whatever you choose to do, don´t rely on a sole ECU update. The drawbacks on this are, as Tobey mentioned, the intercooler temperatures and potentially increased stress and temperatures inside the engine.