Going 20's doesn't really effect performance.. i think these wheels are probably about 5kg heavier (in total) compared to the stock ones i have, so it's really not that much difference. They're also wider which provides more grip, and don't forget the cambering that comes stock, all driving still very smooth and i'm very satisfied with it.
Speedo Calibration? not too sure man. Will have to get that checked, but i don't see a reason why it wouldnt be fine.
Thanks dan!
Also i did watch fast furious 6 last night, and after the movie finish, i went home and tried to catch up to a yellow light, i was going near 100 and did a turn, the car skidded for about 0.3 seconds and then i got it back in control. Can't really blame me, the movie is quite contagious. Hahaha