Your strategy for backing up an 8.5 depends on what stage in your heat you get your 8.5, and whether you're leading or trailing. If you get your score late in your heat, your whole strategy is going to depend on the scoreboard and the clock, but the interesting situation is if you get your 8.5 on your first wave.
If I dropped an 8.5 on my first wave--or early in the heat--and the other guy hasn't had a wave yet and is sitting there waiting with priority, I wouldn't even be chasing another 8.5. What I'd be looking for is a quick 6. I'd be straight back out after my 8.5, and if something else came through as I was paddling back out, I'd just grab it and go chasing a 6. Anything over a 5.5 will work, anything above a 6.5 and you're laughing. You're now suddenly two waves ahead, and they're reading out the scores over the PA telling the other guy he's now behind by two waves. They're reading out that the guy needs a 15, then suddenly he's going, "Shit, he's on 15 and I'm on zero!" I've seen guys visibly deflate when they find out they're now comboed. It's a huge psychological thing and it makes guys either panic or lose hope. Once you get your quick back-up 6, that's when you paddle back out and get priority and wait for the duck's nuts. That's when you start hunting another 8.5. Whatever happens from then, the clock is ticking down and it's your friend, not his.
I reckon your chances of winning the heat if your first wave is an 8.5 is about 80 percent. If your second wave is better than a 6.5 then your chances of winning are about 90 percent. But I've lost heats with 10s before--once to Shea Lopez in Fiji and once to Freddy P in Bali. Neither were fun.