mboy- Just sold my MK indy with a 954 Fireblade lump. There is no way you can say its just for the track. Ok so it had no reverse but other than that I could drive it round town, in traffic and on the track with no issues at all. I did 300 miles in it some days with no problems and returned up to 35mpg a lot of the time.
Even on the track i could get 17-20mpg! No normal car can do that.
And 150bhp/l??? Mine was 170bhp/l.
I'm honestly amazed that you got 35mpg a lot of the time, that's more economy than the bike the engine has come out of acheives (and the Fireblade weighed about 180kg, 1/3 what your car weighs almost, as well as the fact the gearing would be significantly shorter with approx 130mph top end instead of about 180mph). I do find that very hard to believe considering the Westfield Hayabusa my mate had he could not get better than 20mpg, and when thrashing it it was down closer to about 12mpg!
Don't get me wrong, Bike engines in kit cars are a performance revelation, but they do have some significant drawbacks. For those that want a kit car as a pure toy, to drag out on Sunny Sunday mornings half a dozen times a year, they're superb. And for the track enthusiast they're great as they're a very cheap way to lots of performance. But the short gearing, lack of reverse gear, lack of economy, snatchy clutch and lack of flywheel inertia, do not make for a particularly practical experience to be honest.
On the BHP/litre side of things, it's much easier to get more power per litre with smaller capacity cylinders than it is bigger ones. Most 1000cc motorbikes (250cc per cylinder) are up around the 170bhp/litre mark as standard indeed. Most 600cc Motorbikes (150cc per cylinder) are well over 200bhp/litre as standard! Bigger capacity motorbikes like the Hayabusa are down closer to 135-140bhp/litre as standard. Achieving more than 150bhp per litre from a 1600cc 4 cylinder engine (400cc per cylinder) is pretty good going, all the bored/stroked Hayabusa engines that I've seen taken out as far as 1600cc have produced a max of about 250bhp without forced induction! And that's a modern engine design. The Toyota 4AGE was originally designed in the early 80's, and can be bought for peanuts (christ, I've seen entire Toyota MR2's that have failed MOT cos of body rot, but engine is sound, for £200 or less!).
In my opinion, if you want something for road use with a motorbike engine in, get a motorbike!
Anyway, one of these with a 1.4 TDi PD VAG engine, remapped to 110bhp or so (a quick 200 quid job), and probably running through an auto box, would make a great fun green lane and semi capable offroad and dirt track vehicle, and would cost peanuts to run!