I have heard that lighter line gives you more sensitivity. Is that really the case? I'm looking to spool my fluke baitcasting Rod with mono and want to know if lighter line is more sensitive.

It depends on the mono.  Generally the lighter the line, the lighter the rod, the more sensitive the setup will be.  However, mono has nothing on  fluorocarbon or braid (only when it has no slack thrown in) for sensativity. 

4lb trilene and 20lb trilene are made from the same material and have the same properties.

I never really see much difference between say 6lb and 12lb as far as mono goes.  When I step up to 20lb though, I might notice a little difference. 

Has to do more with the density of the line and speed of the rod in most cases. Faster rods that are well built are normally better. There are always exceptions

NO!!!