Ok, so lets say I have saved up some XP and want to increase my character's Might. However, he also has Enhanced Atribute (Might), increasing his might from 6 to 10. Does he have to pay 6 xp to increase the base score or does he have to pay 10 xp to increase the effective score?

You should keep track of your actual score and your ranks in Enhanced Attribute. (Consequently, you will need to also keep track of your "final" score when the two are combined.) And in this way, you can advance each thing separately... your actual attribute or your Enhanced Attribute ability.

Scott

keep in mind.. someone can "turn off" or copy a your powers...

So keep your base and Enhanced score separate

as for spending exp..

Your base attribute is independent of your Enhanced Attribute power (which merely adds bonus ranks to your attribute when active)...

That is what I figured.