The way I understand it, the Outsiders ability now affects ALL modifiers, but what all counts as a modifier? I know replacement values don't count, and powers that say "becomes" but what does?

Thanks.

Anything that causes a combat values to be altered up or down by a specific amount. It almost always uses the word "modify" or "modifies" in the text.

What about things that halve something such as hypersonic range or moving in water?

Reduced by half is a replacement: see the rulebook page 5.
Might also want to note that Outsiders will effect all modifiers when the the new rules come out, but that's not quite yet.

Where I play isn't really a competitive environment anyway. It's really just more of a curiosity.