I'm not sure anyone makes a "kit" yet, but I am sure they will and they'll charge a fortune for what is basically a Lightning/Cobra pulley kit. Maybe we should do this before someone else does....
John Coletti from SVT (now retired I think) indicates that the Ford GT fuel system can handle more hp, he was quoted a couple of times in Muscle Mustangs and Fast Fords when he personally delivered a GT for them to wring out on the strip - and that they did, to good effect producing some high 10.xx timeslips if I recall.
Anyhow, the motor is what we would call understressed, that is, it is built for consumers and carries a warranty. So like the SVT Ligtning and Cobra motors (the supercharged ones, the early SVT motors had some issues) they can handle a fairly good increase in power.
Plus, the GT wears a Lysholm supercharger, not a Roots blower as my truck does, and it can produce more boost than it is now doing without making all that much heat. It is a more efficient design, more expensive too, but peanuts in the Ford GT price. Ford has those set at 12psi, I'd certainly think 16psi total boost or a little more would be in the realm of possibility. I don't know the pulley sizes on the GT, but I'm certain that one of the Lightning/Cobra performance places would jump at a chance to make a set, if they haven't already. I run a mild 4lb increase on the Lightning and we're at 410hp and 503 ft/lb of torque (wheel numbers) - I've got a 8lb+ increase pulley to that we'll run soon and see what she will do. I'm sure the Ford GT will respond well to boost increases!