Your biggest problem, as Jasper points out, is that you have to enter the US with your bags (i.e. go through US preclearance). If you were to subsequently leave, you'd need to go back out through Canadian customs/immigration, go downtown, come back to the airport and go through US preclearance (customs/immigration) again. The US border agents are likely to be unimpressed with that sort of thing, and have every right to send you to secondary screening or even refuse you entry for messing around like that.

If you want to go downtown, your best option is probably to check your bags to Toronto, try to check them in again there for LAS when you arrive (but beware that they may refuse as you're too early for your flight to have opened) and then go downtown. You may need to bring your bags downtown, as I don't think there's a left luggage facility at the airport.

One small chance - you might be able to simply not show up at US preclearance to "collect" your bags for 8 hours. I'd check for someone having done this successfully before, though, because again it's the sort of action the US agents might consider suspicious (or the Canadian security authorities, for that matter) and have your bag pulled/destroyed.