Food for thought:
Just about any distro that isn't "testing" will be stable enough and fast enough to run a web server, so I wouldn't worry about performance too much. What makes a big difference, imho, is how easy it is to maintain -
Are things where you expect them to be?
Are packages available for this distro or will you have to compile from source? Is there an online repository & if so, how extensive is it?
What about packages that are not in the repository - do you have to install from source or can you install from a downloadable package (like an rpm or deb)?
How [well] are dependencies resolved? What about non-repo packages?
Are updates available or do you have to monitor changes yourself?
Do you have to touch the server after the install is complete?
I like Debian because it answers all of the above questions the way I like. This is not to say that Slack or Gentoo are inferior - just that I prefer the deb way.
All that said, the best thing you can do is what you're doing - test test and more tests. My last distro eval lasted 2-3 weeks & covered probably 10 different distros. My desk was littered with OS CDs.