Get an engineers certificate with the 17's on saying they are safer than your 15's and that the extra fat tyres are better for stability or somethin, then show the police next time they pull you over

If they are LEGALLY approved by the RTA, then the certificate will only be used to shut up the police. By right any modifications on your car should be backed up with a certificate, to show that everything is safe and fine and dandy

Hope this helps!