IO540 I think it's more likely to be the dead hand of European harmonisation that is dictating Mode S introduction rather than CAS violations. Does the number of CAS incursions alone justify the huge expense and disruption that introducing Mode S is going to cause? Don't think so.
Mandatory Mode C would be sufficient to cover every complaint I've heard about VFR traffic. I talked to a chap at the PFA Rally and for 1,500 quid I can buy a Mode C transponder (1,000) and a new 760 Radio (500). Another 500 squids to fit them (they are both instrument hole mounted) and my little Auster would be seeable on everybodies TCAS / Radar / Crystal Ball etc. I might have to get batteries with more oomph than the pair currently powering my radio though but then again, maybe not. He did say that the units were very low consumption but he would say that wouldn't he!
IO540 reckons that if I have to fit Mode S then it is going to cost me 3k plus installation. The extra 2,000 for the Mode S system will provide no significant improvement over Mode C as my steam age (literally) aeroplane does not have the electronics to transmit heading, height etc. from the aircraft systems unless the CAA approve connection of things like a Pilot III to the Mode S.