If he wants 17s, let him buy 17s.
16s generally handle better because most people fit cheap-ass heavy wheels...and the 16s are not as heavy as the 17s...so the handling improves.
Fit a set of strong, light 17s and 205/40/17s and it will handle just fine and the speedo be perfectly usable...but the longer gearing will not do the already comically long gearing any favours...so the car will probably be slower at accelerating...but much is down to weight and width.
Fit a set of light & strong 16s and with the right tyres, it'll accelerate better and handle better...but fit some stupidly wide heavy-ass 16s and you bugger things up too.
Having tried 15s, 16s and 17s on my Golfs, I think the best balance is light & strong (BBS RX) 7x16s...but the offset, width and weight come into play too.
It's not black and white.
