I have just spoken to Steve again regarding some further mapping work on my car and general help regarding fuelling etc.
I have to say that he is one of the most knowledgable people and genuinely helpful people I have ever had the pleasure to work with on the car.
But I would say to those fitting GEMS etc. - like any tuning, don't expect instant magic. Steve can quickly map your car to a safe point with good power - if you want to extract the ultimate, and get it to the limit - that takes more than a few hours mapping and is hard to do on the road anyway. Same as any other kind of tuning. Every car is different too, and you need to make sure the rest of the car is up to it, so you don't trip up on other problems. I reckon after the next mapping session my car will be bang on, and probably will sport an additional map for 102 race fuel.
having spoken about that it could give 20-30% more power and torque. Ok, it is pricey, but hey - worth it!
Steve also mentioned that Optimax |PLS| Millers CVL allows a few more degrees of advance too.