on a mk3 if you're running 16's then the profile should be 45. If you're running a 40 profile then you're speedo is out in the right direction. i.e you're going slower than the speedo says you are. very slight improvement on acceleration and a slightly lower top speed. shouldn't make much of a difference to brakes and i'm guessing by the fact you're running r14 wheels that it's a slow boat you've got