Thanks Harkins - been meaning to try out the new relic gem, but that rails-footnotes plugin looks pretty cool. I think someone in my local Ruby group mentioned it at a meeting and I forgot all about it.
I actually did try sinatra via metal prior to going the PHP route. What I found was that I was either continuing to use ActiveRecord, which ended up killing some of the speed benefit, or I was trying to get it to do direct SQL queries, which is actually kind of a PITA when you're not just using AR. Also, all the metal tutorials and examples out there are useless

Since I only need to make two or three queries in each call, I took a stab at it in PHP, and it worked out for me.
I should probably make note that although the PHP did actually perform faster on those small scripts, I wouldn't normally condone that behavior from the beginning. Pre-mature optimization is definitely evil. At this point, my combat code is pretty stable (2+ years in the wild) and not likely to change drastically on the front end any time soon.