7.11 Member Scaling Capability revisited

I've poked around the forums and see references where people have 6k, 10k or more members on their sites. Cool.

When I researched to see how many concurrent users someone may have it seems it's 20-30 at any given time.

I'm concerned about the performance when there's 100 concurrent users or more during a 'spike'.  I've upgraded my hosting to cloud with 4gig, 3.16mhz quad procs and 3gig bandwidth and turned off blacklisting yet pages still are a bit slow to load when I'm testing only 4 concurrent users. 

I don't have any testing software to simulate a user spike and wondering if anyone has ran those numbers on 7.11?  Say, 1k users on at the same time?  I would like to know a 'sweet spot' of forecasted number of concurrent logged in users for a given hosting/hardware specification.  Yes, it varies but there has to be a rule of thumb, surely.

PS, I have all caching off right now so I can easily edit things. Also, I have not loaded any PHP accelerator.  Unless those boost speed by more than 15%, I think it may be too slow?

http://www.boonex.com/trac/dolphin/wiki/TutorialHowToCalculateLoadAndHardwareRequirements

Some tool for site load testing:

http://loadimpact.com/

Just FYI for anyone interested - although I've not tracked down why, I think some of the performance issues actually has to do with the connection between the client and the host. 

Using various machines on different IPs in different locations within a 5 mile radius (served by Verizon FIOS by the way), there are times the site loads pages including cPanel very slow.

But I can call friends a couple of hundred miles away to pull up pages and the site performs as expected.

Weird.  Other sites are not effected except maybe FB or Hotmail at times (really slow to load).  You think this could be a load problem with the ISP or whatever logic is deciding the hop route to the server?  Tracert results sometimes show timeouts but other times they look acceptable yet the page(s) load slow. 

A mystery.