Looking to purchase my first custom ES-3, acoustic. Had a Godin 5 th Ave before now switching back to a lefty. Looking at a mahogany or elm body, being new to this I could use some knowledge!
Although elm is listed as an option wood, I've never know anyone order or make one, and I've been around on the forum for a few years now. So I can't advise you on what that'd look like. You'd need to email PBG anyway to discuss a custom order, so they may be able to tell you more (though they may never have seen one either if they haven't sold one).
I've seen people saying they were intending to get a mahogany ES3, and there was a brief spell when some were available direct from the PBG store without having to custom order, but I've never seen any finished ones, and have no idea what they'd be like. It will be a mahogany ply, not solid mahogany, which will need more work than the standard basswood ply model in trems of grain filling. Sanding the grain filler back may be an issue because of the thinness of the outer veneer layer. I have no idea what the external ply layer will look like on the mahogany model, but I know that the outer ply/veneer layer on the Basswood kits is nicely figured and looks very much like plain maple does (no flame or quilt etc). The darker nature of mahogany will limit your choice of finish colours compared to the much lighter basswood. But you should be able to manage a mid-brown to dark brown/black burst as I've seen on some of those Godins.