I have no hard evidence but on the one stretch of road that I know well and on which the bright yellow boxes have been replaced by average speed cameras there has (in my experience) definitely been a massive reduction in speeding (it is a 50mph dual carriageway on which it was hard to do 70 without being tailgated by someone who wanted to go faster).
It's also deterred the idiots who would race up towards the cameras, brake hard for a couple of hundred yards and then accelerate again.
On the other point, I'm not sure we will see all vehicles fitted with tracking devices in the foreseeable future. In any case, without enforcement, they will be as effective as speed limit signs and, I imagine, disabled by the more determined.