Heh. I don't think I have ever used it. But I have used the helmet mod that gives you bonus experience.I forgot about the elixer!
The "XP per level" only increases up to something like level 60, so it doesn't really slow down from there. Zeds get stronger, but stronger zeds give more XP; so as long as you can hang, the progress should be pretty decent.And I'm lvl 77 or something. Obviously, it get slower to level as you go higher.
That's gonna be like a year!?!