Gideon stated here that Q2A has been tested with hundreds of thousands of posts and users. However I'm not sure if it's been specifically "stress tested" with many users at once.
If you're able to do some testing and profiling yourself, let me know of any specific queries or functions that can be improved.
You mention "complex queries" - there is the option in qa-config.php to optimise for a local database (if MySQL runs on the same server). That simplifies the queries greatly.
Make sure to check which queries are slow, they could be from plugins.
By the way there are some big optimisations coming in v1.8. Depending on certain setting you could see page load times 3x faster. And there will be caching, to lighten the load even more.