You confuse purism with pragmatism. As a gearhead and enthusiast of 40 years, I have learned what works and what does not. I am not saying "Don't run non-stock wheels." I am saying "Don't run 17" wheels on that particular car." Have you ever learned about tire contact area? If not, you have some research to do before we go any further with this discussion. It's interesting that you mentioned rotational inertia instead of acceleration, and you're correct with that more precise term. Now, think about it: not only does the larger wheel/tire accelerate slower (which you know), it also brakes worse for the same reason. So you have a disadvantage at the beginning and at the end of every straightaway. I spent a decade autocrossing regularly (still do occasionally - look for last year's O'Fest results to see how many other 5ers I outran) and I have been an HPDE and car control (e.g. Street Survival) instructor since 1992. I think I know what I'm talking about. OK; if there are too few 15" choices, why jump up two inches in diameter? Do you think you'll find a better tire than my 16" Yoko Advan A038s? ... unless you autocross. I've won and lost events by hundredths of a second. A bigger wheel/tire combination only means I would have lost more. I also have direct experience with different size wheels and tires on the same car. I have run 14", 15", 15.35" (TRX 390 mm), and 16" wheels on 3.5 liter E28s. While there are quite a few guys who think like you ignoring advice like mine and trying the same thing on their E28s, there are none who later deny that the advice on heaviness in the wheels is correct. It is a tangible feeling of clumsiness of the car that goes away when they go back to a smaller, lighter wheel/tire, regardless of the sizes involved.