@adelevie: I can't give you any useful information about performance as a) i don't have any benchmarks and b) I don't really care because this setup is meant for development primarily.
I assume though that the performance should not be a problem, as
a) It's rack, and rack is fast
b) you can configure which rack server to use
c) it only serves static files, so there's hardly no overhead
d) the bottleneck will be HTTP and the network
That being said, I don't think that you'll have any noticeable performance differences between serving from s3 and serving from rack.