you don't want heavier tires and defend using spacers?
In all reality, there isn't much weight difference between 245 and 255. It is one pound. However a .85lb spacer fitted to the hub is still different from one pound of rotating mass 13" away from the hub. This is also why lighter tires are really more important even than lighter wheels. It is the location of the extra weight that matters most. Adding a spacer is no different than a little more metal on the hub.
245's push on this car. 255 better, 265 even better, 275 perfect.
This is subjective and depends on your intended purpose. Sure, for the track, you want maximal grip. For street driving, however, I prefer the sharper steering response of the narrower front tire. Wider tires have their downside too. More noise and more rolling resistance, especially as the tires wear. Still, a 265/35/18 and 285/35/18 is probably what I might consider the best overall set-up for an M3, all factors considered. I almost went this route, but I spent the money on my ZCP wheels and didn't want to just ditch them.
conti's are slower but not dangerous at all.
No, not dangerous. The DSC saves you, but it was kicking in all the time, constantly cutting power, and with the DSC off there was less of a safety margin than I cared for on the street with its associated street hazards.
i love PSS, but wouldn't swap until use up current tires. if buying new ones, no way would i pick oem sizes. too much understeer!
Again, personal preference. I find the M3 nicely balanced for spirited street driving. The steering is perfect for my tastes.
Over the years, I have seen too many people mod their cars to become more and more like pure race machines when this isn't how the cars are really ever driven. Maybe a few track days here and there. I encourage people to think about what will give then the most driving enjoyment in their real world driving circumstance, and not what might give them the best lap time on some isolated track run, or what exhaust will get the most attention at that random once-a-year car show.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner