The trick to going fast in a 2cv/Dyane is accepting the crazy lean angles, driving smoothly and carrying all the speed you can muster through the bends. Simply
great brakes help enormously too.
As for 17" tyre choices, you could do worse than speaking to Longstone tyres -
http://www.longstonetyres.co.uk/search
They have a fair selection and have been in the business a long time.
Oh, and Micky, it sounds like your traffic cop used some good, old fashioned discretion in dealing with your bonkers 80+mph careening around in a Lomax. Mind you, I'd have probably given you the ticket...