I Don't think Fi cars are a good comparision ,As Peter states the larger tyres create a much larger rolling circumference, so our 8's guys just don't worry what the 8 series Police say,it's your car, & yours to do with what you want , personally i like 19's [uber cool ]
I bought my car with 16's on ,they had to go, then alpina 18's but when i lowered the suspension they somehow didn't look right, 19's & lowered suspension= Verrrrrryyyyy Niiiiiicccccceeeeee