This guy is seriously worried about your speedo being out by a couple of mph? This is the problem with mechanics that work on run of the mill crap all day, they are instantly horrified by the idea of someone maybe putting a non factory spec'd tyre on their car !!
Your speedo will be wrong from factory anyway [they all are], so all you have to do is check your speed on a GPS app and see where your speedo is at 30mph etc.
You will need lower profile tyres for sure to fit under the arches, as you've found. As always, a search of the interwebs to find pics of cars that look how you want yours to, then see what size tyres they have. Having never run 17s myself I can't advise, but on a 16"x7" rim I always go for 195.40s as these look good.
I presume you are lowering the car too?