Giles' conclusion to go with Nginx combined with Lua is a great one. At 3scale we find the combo terrific for the purpose of a high-performance API Gateway:
<a href="http://3scale.github.io/2013/01/09/augment-your-api-without-touching-it/" rel="nofollow">http://3scale.github.io/2013/01/09/augment-your-api-without-...</a>