Hi from England. I know SOME about cars, tyres, suspension, etc., but not too much yet which is Altezza-specific. All I will say is that, to my eyes, folks do some really odd things where wheel & tyres are concerned. The Formula 1 guru Gordon Murray (designed the MacLaren F1 Roadcar) tells us that our wheels should be JUST big enough to clear our brakes - and that anything else will only serve to put more mass farther out from the hub-centre. Now decades back, Colin Chapman of Lotus declared that weight is evil - in his words, "Add Lightness" - that EVERYTHING on the car should be pared down to the bone - ok, it NEEDS these suspension arms, but instead of them being 18mm diameter, will 16mm work? If 16mm didn't break in testing, then 14mm would be tried - in a stronger alloy if need be. "Give a car a big engine, it will be fast on the straights. Make a car light, it will be fast everywhere". He was also anti-washer in his philosophy. "We would never put a washer under a nut. Who wants to drive a washer around a race-track?" It may sound like saving grammes, but it all adds up.
OK, so keep it light is what Colin said. Now what Gordon says is that ROTATING WEIGHT is not just EVIL, but is SUPER-EVIL. Every gramme of your wheel/tyre combo must be accellerated from rest, must be steered to change direction (the more mass you have, the more inertia will arise as understeer) and must be braked to a stop. It's easy to see how you could end up chasing your own tail, fitting larger, ever-heavier brakes (with ever more mass in them!) to try to control this dead-weight express you have built. My Altezza is the first car I've ever had with 17" wheels on it. I shall see if a spare set of my 15" MR2 wheels will fit whilst I refurbish the 17"s, which have some very minor kerbing. And if the car feels better on 15"s, then I'll keep it on them. These days, FASHION is the reason cars are wearing wagon-wheels. Tall wheels raise the car from the ground, needing the suspension to be lowered past design criteria, and sprung over-stiff to prevent it bottoming-out at the depth of travel. I've seen young guys over here fit a car with 17"s and wonder why it's suddenly on stilts, and you can easily put your fist on top of the tyre and still have clearance under the wheelarch! Something called "Mathematics", and you don't wanna be caught on the wrong side of that line.
One more point about your car - ground-pressure. Point loading of the mass (I am guessing you are stripping out some weight?) is shared over the area of the tyres' contact-patches. It becomes obvious that these have a shape, which is pretty much oval, running across the tyre tread and narrowing at the outer and inner edges. Consider that a smaller wheel/narrower tyre combo has an oval which can be MUCH fatter, front to back, than the contact-patch of a very tall/very wide tyre - remember it's the same MASS squashing the rubber down on the road, but spreading it over this tyre which is a few inches wider, we are looking at a very wide, very narrow print of contact. THIS is evil, as in effect, the TIME that the driver has to sense break-away in a bend (when you need to catch the car and apply opposite-lock) is dependant on the LENGTH, front-to-back, of these contact-patches. Long-patch = more time to feel it all going wrong = more time to correct = driver lives to complete lap. Super-short patch = almost zero time to feel this tiger turning around to bite you = no chance to correct = this 'race-car' built with all these great goodies on it slams into the tyre-wall having rotated in the blink of an eye. Driver not so happy.
The first car a young friend of mine got was an older VW Polo. It had THE skinniest tyres on it, they were 145-section! I test-drove it when he was looking to buy it, and found that yes, they are skinny, but they load up nicely. He bought it, and a few months later fitted some wide wheels with 195 tyres on it. I drove it again, and it was utterly ruined - zero response to throttle, a habit of vaugely slipping sideways at slow speeds in bends, and hopeless in the rain where water couldn't disperse from under the contact-patches. An extreme case I know, but in accord with the principles I stated.
I will put my hands up as "Guilty", having done almost as bad myself - A WEEK before I read the big article by Gordon Murray, I bought some wheels for my Alfa-Romeo 75 TwinSpark. This is supposed to be a light, fluid-handling saloon-car, which has great handling balance, as the gearbox is on the back axle, in common with cars such as the Porsche 924,944 and 928. I was running 15"s with 195/45 tyres, and changed to 16" wheels with 205/50's. Oh dear - immediately, the house-fly like responses of the car were dulled. It was now slower off the line, tragically so. Bends were met with a dead split-second from the steering wheel, as it pondered how to change direction of these two larger, heavier flywheels out in front. Braking is definitely worse than before, and it was never good to start with! I'm going to sell the 16"s on, and go back to 15"s - lesson learnt. I can even fit a bigger BMW disc under the 15"s on the front of the car - so there is no excuse for me not to stand by the wisdom of 1) What I've read as the wisdom of greater minds than mine, and 2) What I've had empirically proven to me in the Real World, after wasting my own ca$h running up a blind-alley. In short (Oh come-on, I don't do short) what are the smallest wheels, with the narrowest grippy track-tyres that you can run on your Time-Attack car? Please, do a run on them, and see where you stand against going super tall & wide.