What is causing the limit of 300 connections per second in Apache 2.2?

There are all sorts of system-performance-related possibilities, but the fact that the limit is such a nice, round number is rather suspicious -- you wouldn't expect such a neat limit if the bottleneck were, say, CPU or disk I/O (or even network). What's even more suspicious is that it's per second -- if it were a limit to Apache or MySQL, you'd expect a limit on concurrent connections or requests, but a limit of 300 per second sounds like something else.

What is sitting upstream from you, network wise? If there's any firewalls, load balancers, DDoS mitigation systems, or other L3+ devices up there, I'd take a poke at them to see if there's anything in there that could be causing it.

Do you know what anyone getting above your 300/second limit sees? (You might have to hammer the site yourself a bit to see what happens) Are there log messages anywhere that might give a clue?

After making the configuration changes did you do a graceful restart or shutdown/start? Changes to ServerLimit will not take affect if you do a graceful restart. A complete shutdown and start is required.