What's the most scalable and high performing Amazon Web Service (AWS) configuration for a RESTful web service?

1a) Nginx is asynchronous server (event based), with single worker itself they can handle lots of simultaneous connection (max_clients = worker_processes * worker_connections/4 ref) and still perform well. I myself tested around 20K simultaneous connection on c1.medium kind of box (not in aws). Here you set workers to two (one for each cpu) and run 4 backend (you can even test with more to see where it breaks). Only if this gives you more problem then go for one more similar setups and chain them via an elastic load balancer

1b) As said in (1a) use elastic load balancer. See somebody tested ELB for 20K reqs/sec and this is not the limit as he gave up as they lost interest.

2a) Host static content in cloudfront, its CDN and meant for exactly this (Cheaper and faster then S3, and it can pull content from s3 bucket or your own server). Its highly scalable.

2b) Obviously with nginx serving static files, it will now have to serve more requests to same number of users. Taking that load away will reduce work of accepting connections and sending the files across (less bandwidth usage).

2c). Avoiding nginx altogether looks good solution (one less middle man). Elastic Load balancer will handle SSL termination and reduce SSL load on your backend servers (This will improve performance of backends). From above experiments it showed around 20K and since its elastic it should stretch more then software LB (See this nice document on its working)