Best approach to benefit from CSS performance enhancements

So file structure is just for organizing and not taken into account to determine which content gets fetched for a certain page type. That's already helpful.

I am currently using two types of bracket classes around page specific statements and I was wondering if both work well together with the CSS performance enhancements. So is there a difference between

.ForumPage {}

and

#lia-body.ForumPage {}

?

I often have to use the latter to reliably overrride Khoros base skin statements.