I'm going to be re-installing my intake manifold, lower and upper plenums in a couple days, and although I've never used it before, was wondering if using a spray-on gasket sealant would be benificial, in addition to new OEM gaskets.

This is what I'm talking about:

 

It's not a bad idea, but I never use it.

 

If you tighten everything the right way and are using new gaskets... I can't really see any benefit from using that stuff.

Boosted... who knows. But even 15psi is not very much pressure on your intake manifold gaskets... I run 80+psi on ghetto *** rubber o-rings in my lab without leaks, I don't imagine leaks with 8-20psi (whatever boost you're at)