[Matthias Rudolph]

How well policed is track-limits, and what is done about infringements?

Absolutely not policed, and nothing is done about it.
You can try to create threads like this one: http://www.simracingsystem.com/showthread.php?tid=2648
But it won´t change anything.
The people who are cutting on purpose obviously read it, but no one ever replies.
You can try to protest them, but I guess nothing will happen.

Watching the TTcup race at Spa earlier many cars take liberties in the section approaching Stavelot, and exiting the left-hander and heading towards Pouhon.
Some say there is no gain, but surely there is, because if they attempted to keep within track limits on the wrong trajectory they use, then they would lose time because of the necessary lift off throttle and the subsequent loss of speed in the following fast bit.

Just tried it in my first race this week.
If I follow the Tracklimits I´m getting a 6-7 tenth disadvantage against people who are cutting the exits in the corners you named.
Tested offline under same conditions.
I don´t wanna be the Idiot who´s cutting on a competetive sim.

You will never get rid of the cheaters, and no one will ever reply to such threads.