I am building a GAE site that uses AJAX/JSON for almost all its tasks including building the UI elements, all interactions and client-server requests. What is a good way to test it for highloads so that I could have some statistics about how much resources 1000 average users per some period of time would take. I think I can create some Python functions for this purpose. What can you advise? Thanks.
If you have the budget for it, a professional load testing tool will save you a lot of time and produce more accurate results.  Some of those tools handle AJAX apps better than others. I will naturally recommend our product (Web Performance Load Tester) and one of our engineers will help you get it working with your site. You should, of course, evaluate other products to see what works best for your site. Load Impact and Browser Mob are online services that in many cases handle AJAX better than the more traditional tools (except ours!), but they also have downsides.
How to test my GAE site for performance