Just curious in getting a 4.11 diff gear ratio setup put into my ba & was wondering on the cost and effectiveness of this mod.

Mods at the moment. High flow filter, Pacemaker extractors (low to mid range), 2.5 Redback exhaust with a 2.5 High flow 100cpsi venom cat converter & the standard 3.45 /3.46 w/e the ba's have.

In the future my ba xr6 will receive a flash tune if that also helps for information

I remember putting my 3.45 LSD into my 92 Ford eb awhile back and i know that diff did wonders with improvement but i have never gone upto a 4.11 which is alot more down low & im curious how the car will boogie overall..what power would i lose top end..i dont mind losing some if its over 150Kmh..just after more oomph down low but not lose a good ammount of power from 100Kmph to 150kmph. Any thoughts?..

You won't gain or lose power. It will just be a crap load quicker off the line in general.

You're going to have the same issue that I had, and that's getting gears for the M78 diff.
Try contacting Malwood Automotive, and see if they have any available for the diff, but don't hold your breath. I'm getting a place to source some gears from an AU and shimming it, and that's just for 3.9's. The shortest ratio the M78 can get is 3.73, which will still be nice off the line.