apparently it makes the neck stronger but I doubt there is any tonal difference that is noticeable .... all woods of the same species are gonna sound a little different anyway
Stability.
The roasted necks are supposed to require less maintenance, but Suhr guitars with regular maple necks are almost maintenance free anyway. Have both and no problems with either one.
I have owned both in five Suhrs. I think the differences I heard were more from or, all from, the bodies. The guitars I had with roasted were Alder and the regular maple basswood. The guitars with the roasted necks were smoother sounding. The roasted necks had a softer feel. Like butter. Felt great. With that said I did not prefer the smoother sound. My experience and really doesn't hold too much value without comparing a roasted neck and basswood body.
A mahogany neck. If Alder or Basswood....I'd get roasted maple. The last Suhr Modern I ordered was with a Basswood body (flame maple top) and roasted maple neck (rosewood board), and I love it. If I were to ever order another one, I'd get roasted every time. I love the darker look and the stability. For the small upcharge they want for roasted, it's well worth it IMO.
My 3 moderns have mahogany bodies with mahogany necks. I do have one coming in that is basswood body, maple top, and maple neck that I'm anxious to try.
I have a Tom Anderson Drop Top with mahogany body, maple top, and maple neck that sounds great. I do think that my basswood body, maple top, maple neck Drop Top did sound better though.
I believe that John Suhr's preference is basswood body, maple top, with maple neck.
I've had experience only with one roasted/carbonized maple neck, and it wasn't a Suhr (but still was a high end manufacturer), so take this with a grain of salt. But that neck was riddled with deadspots. Maybe just a fluke, but enough to steer me clear of ordering a guitar with a carbonized maple neck until I can check out a lot more to find out. I've read folks claiming the carbonized maple is more resonant etc., and if so, that could explain lots of deadspots (resonant neck is all well and good unless it wants to resonant at the frequency of the note you're playing). My $.02 anyway.