You have to be really careful when thinking up incentives, because they can easily backfire and make the problem they were intended to solve worse rather than better.

It's like that story about trying to get the rat population under control during the French occupation of Hanoi. The authorities incentivized citizens to kill rats (by offering them a small reward for each tail they brought in), and although it did work for a short time, citizens then started leaving the rats alive (after taking their tails), hoping that they would breed and produce more tails for them to collect the bounty on. There was even rat farming going on, to take better advantage of the rewards. Authorities realized what was happening and stopped paying for the tails, but the damage was done and they now had an even bigger rat problem than when they started...

If you add a financial incentive to getting your post in early, then I think that would ultimately decrease the quality of the first few replies. There's already a kind of "traffic jam" at the beginning of some topics (usually questions) with the first few replies trying to compete for merit. I often don't bother to post in topics like that because it would take me too long to do the question any real justice, and I know that by the time I'm finished with my reply there'll already be a bunch of (pretty lazy) "fast" answers that (taken as a whole) cover most of what I was going to say. It would actually be pretty cool if there was a way to call "dibs" on a topic and stop other people from posting "rushed" answers while you work on a comprehensive one.