When it comes to booms and mast extensions, what I wonder about is, is it better to have X amount of pole in there?

EXTENSIONS:

For example, if I use my 48cm extension in a 430 mast, and I only need 2cm or 6 or 8cm of pole in there, when is too much pole too much?

Can this cause mast snapage?

And the other way around - some folks tell me they only use a mast cup(?) on occasion with no metal extension, or only have a small amount of cm up there??? Will this hurt things?

You want to right bend, but safe bend right?

BOOMS:

You have a sail that you could use either your wave boom or your slalom boom - do you go for more pole in the hole with a slalom boom, or have the wave boom almost completely out and possibly its bendy?

Help!

I should write for the MX

I've always been told not to put all of it in but that's another story for another time

Mast / extension: too much extension up there makes the bottom of the mast stiffer and you can notice the sail set a bit poorly. Can't see how it would cause a breakage, unless the extension jams at the top end due to mast taper and downhaul is forcing it into the mast. If it inserts by hand it's fine.
Too litttle inseertion is asking for trouble and can cause breakage. Eg drilling another hole in it to get another 2cm extension is bad bad, there is a reason the holes don't go all the way to the end!

Boom: the less extension the better as it stiffens the boom a lot. If you can use a slalom boom with no extension that would be waaay better than a wave boom fully extended. I'd never sail waves with full extension - buy a longer boom.