I remember that, as you've written, you've ridden racing bikes and, IIRC, you also raced in professional classes, so you're used to high level performances.
I use a 2008 SM 610 on tracks only. At about 8.4k miles (8.3k on the streets and 100 on tracks) I disassembled the engine because of a certain problem I caused during maintenance; it was my fault. I checked the piston rings and they were almost in the same tolerances of the new ones. I just replaced the timing chain.
Now it has more than 10k miles and, since that disassembly of the engine, I've used it only on tracks (mainly on asphalt), using sportive oriented but street legal tyres. They say that on the street you can cover about 20 to 30k miles before rebuilding the top end.
JonXX covered dozens of thousand of miles with his and, still IIRC, he replaced the piston at 54k miles!.
Anyhow, I never practised motocross (I'll hopefully buy an MX bike soon) and it feels quite clumsy in the dirt. Two friends of mine, who have proper mx and sm bikes (450s for sm and 125 and 250 2 stroke for mx) tried it; one of them in a mini mx track and the other one in an sm track without dirt section: they both say that it really feels like a slow tourism bike.
I tried one of their 450 KTM on the asphalt and it felt better than my bike, although I didn't want to risk too much with another person's bike on the pavement. I also tried one of their mx bikes: a 2000 Honda CR 250, in an mx track. It really felt better than mine.
My opinion is that, given a certain rider used to a ride a 450, he/she will have almost the same fun on the asphalt despite the fact that he/she will be quite slower, while in the dirt the rider will have far less fun and the laptimes will be much affected. The problem I feel in the dirt is that, probably because of its weight, when it starts to pitch/roll/yaw because of the hurdles/holes of the track, it really feels hard to control, even if you squeeze it properly between your legs like a pro. The track I've ridden in most has a
"Sky Section" made up of asphalt in which the bike feels way better.
Besides, the carbed model, at least here in Italy, has about 5 hp more, according to the registration certificate, supposedly because of pollution related restrictions.