Never heard of gatling either. Thanks.
How many local machines did you need to hit the "breaking point"? Seems like local bandwidth / available machines etc. could quickly become a bottleneck.
At first those 4 machines were hitting the database so much that it was the bottleneck, what we did to solve that was to reduce the number of accesses done by the front machines using a memory cache. It can increase the latency when loading the cache but works fine 95% of the time.
Another option for us could be a larger database machine, but we were using Amazon RDS ... believe me you want to keep that machine as small as possible or expend a lot of money :-)
We used two machines to simulate up to 600 requests per second, that was much more than our average use, actually much much more. And we measured it without HAProxy or Varnish or similar systems between our servers and the clients.