I just went through a load of investigation on wheel/tyre sizing...
I just replaced my wheels for winter; had 215/40 R17 for summer, now have 205/50 R16 winter wheels/tyres. I had called SEAT to check their recommendations - they said they had none(?), called the Insurance co. and they said as long as I told them I'd changed, they were happy.
Saw on another post that a guideline is as long as the difference is within +/- 2.5% of the original, all is well. Your setup will be -1.88%, so no
problem.
205/50 R16's will cause your speedo to read 30MPH when you're really doing about 29.5 and will increase your ride height by about 6mm.
You can get these calculations from
http://www.willtheyfit.com/.
205 and 215 tyres will fit a 6.5" or 7" rim as standard (no
problem), see below.
Rim Width Min Tyre ideal tyre max tyre
6,5 Inches 185mm 195 or 205 mm 215 mm
7,0 Inches 195 mm 205 or 215 mm 225 mm