Mellors Elliot are involved in some pretty big power F4Rs at the moment, valves were one of the big limiting factors they found I believe, friend of mine knows a lot about them, I'll tap him up for what ever info he is allowed to share.
I beleive Jason Hellier built on at that sort of power recently too for rallying, one of the things he had to do was changing from AT Power bodies to jenveys as the ATP just werent getting the fuel and air to mix as well I believe and it was costing him power.
There are a few people out there doing it for motorsport so its not anything massively difficult, just expensive and obviously pointless for a road car as the engine wont last long enough and you'd be so much better off spedning a fraction of the cost and having the same power reliably from a turbo instead.