I’ve owned a handfull of Jazzmasters and from what i can tell the only big difference in the spec is the colour scheme. Both guitars have a TOM bridge which is the major difference from a stock Jazzmaster. I know they slightly moved the trem forward on the classic player (to eliminate the need for a buzzstop), not sure if they have done this on the Mascis sig.
Most people i have spoken to have said there is nothing particularly special about the Mascis sig other than the fact that that is what it is.
Personally, i would love one just for the sake of it.