We live in Maryland, house built in '75. Doing some remodeling and have removed drywall on the exterior walls in a couple rooms, will be doing more. Walls currently have R11 fiberglass and in a few spots it needs to replaced regardless due to mice and a couple other issues. My question is would it be worthwhile to just replace all of it with Roxul R15? Exterior was just T111 plywood, but we added insulated vinyl siding. We did have an energy audit done and followed up with quite a bit of insulation and air sealing work (done by a contractor). Thanks for advice.

Yes, anytime you can improve the envelope, it is worth doing.

Going from R11 to R15 in the stud cavities might add about R1 to your overall wall assembly R value due to the low R value of the studs that provide a parallel heat conduction path. I would fix insulation that needs fixing, but wouldn't do a wholesale replacement. You would get more overall R/$ and better air sealing if you installed 1/2" rigid polyiso insulation on the interior side of the studs and taped and sealed all edges prior to hanging drywall. With this approach, you would need to install mud rings (extensions) on your electrical boxes and would need to shim out window and door trim by 1/2" also (easily done if you have a tablesaw to rip lumber to the desired thickness).