The ISF wasn't designed to chase bigger HP. The simple fact is Yamaha have already squeezed out pretty much all of the extra performance available from the engine.
If I was still looking to chase big HP figures, I would have spent the £20K on a single turbo Supra pushing out anything up to 800HP!
The ISF isn't a car you can tune on the cheap unfortunately. My intention is to only do the exhaust (got the same Takeda intake fitted as Stu), and maybe the headers. The company that sells the headers (PPE), and the US forum suggest a gain of up to 46whp without cats (33whp with high flow cats). Not sure how they only got 20HP with intake, exhaust, headers and a map?! Seems low to me.
The other areas I intend to focus on are: suspension - going for the KW version 3's that the US rave about, brakes - will stick with the Brembo discs for now, but want to try the soon to be released full set of Hawk ceramic pads, interior - Going to have the silver fake carbon trim re-finished in real carbon, and a couple of bits painted in body colour. Will also have the engine cover painted in body colour to match. That awful rear centre armrest is also being re-finished in carbon.
I'm half way through fitting the Apexi throttle controller (got good reviews from the US folks), but will be getting a proper auto electrician to wire in the reverse cut-off (don't think anyone in the States bothered, even though it states in the installation guide it does affect the functionality of the unit!).
The alloys are going to change to the 20' Ace Mesh-7's with a full set of Vredestein tires (show me one other ISF in the UK with non-OEM alloys!). These are not only bigger, better looking (my opinion!), but weigh less than the OEM alloys. At under £1K (provided I can bring them back on the plane! Lol), they are really nice.
I've got some paintwork I'd like tidied up (showed Stu the chipped doors from the last owner!), but other than that, no plans at the moment! ;)
The HP game is best played with something that is FI to begin with. The Supra is a perfect candidate for chasing big numbers, as it was de-tuned to meet the gentlemen's agreement in Japan (cars were under 300HP), but had the 2JZ-GTE block, that can handle double the power and more, with stock internals!
The fact is it will never be as refined, comfortable, practical, or exclusive as an ISF though.