The JIT localization is a bit tricky. I don’t know if it’s worth the trouble – it’s not that much of a performance gain – but I’ve started it, and now I want to make it work ??
The best solution would be to patch WordPress itself. But that’s no option for a plugin (but I’m working on some patches which might find their way into WP some time).
The current JIT implementation overrides the WP_Scripts class and works much better than my first implementation. Also it’s less likely to cause errors because if somehow WP_Scripts is used first (which should be prevented by WPPP in that it tries to be the first plugin thats loaded) JIT won’t be enabled. I plan add an indicator if JIT is enabled in the next version.
That said, if JIT causes you trouble, keep it deactived for now. The main performance gains come from MO-Dynamic or native gettext.
And if you have any feature suggestions for future versions of WPPP I’m all ears.