The label "all-star" means nothing. Howard was playing the best hockey of his career early this season for sure. But he has a larger body of work and he is nothing more than a solid goalie that has issues with the shootout and consistency. And the shootout does mean something - it sure would be nice to have a couple extra points from all those shootout losses.
That being said Mrazek has shown enough that I would gamble and see if Jimmy would waive his NMC. He's a lot of salary for someone who isn't the clear #1 on the team and I think you could bring in a veteran for insurance like Jhonas Enroth at a much lower cap hit and use the $ elsewhere on the roster.