I think the problem with the "Pay for Search" concept is that since search is such a basic, essential function of a BB, you'd end up with a ton of people paying and put yourself into the exact problem you're having now -- too many searches being executed simultaneously. Yes, you'd have more cash for more horsepower, but you'd also be losing out on ad revenue.

Personally, I'd be looking into fixing the (fairly inefficient) way vBB does searches, or perhaps a search appliance that would allow you to tweak the indexes so people would be able to focus their searches a bit better -- by username, forum, etc. Wouldnt be as seamless as it is now, but that can be fixed with a bit of re-coding.

I've handled various extremely busy BBs for entertainment companies in the past and have been in this very same situation. There's no real easy fix without a lot of cash being thrown around. Growing pains..