I am one of those 12-stringers who plays in standard tuning.I have a Gibson B25-12N --the smaller body version of the one Gordon Lightfoot plays. I do prefer the tone it projects at standard pitch.
I bought it in 1962,and there have been no problems with keeping it in that tension.
But it IS built like a brick commode. There is very little dip or hump at the bridge; much less so in fact than my 6-strings. It has a 3-ply bridge plate 50 % thicker than normal (in a 6 string). It also has the infamous adjustable Gibson saddle, that can be raised or lowered from the top,with a screwdriver--there are 2 threaded bolts,plus 2 non-adjustable smaller bolts attaching it to the top. I think this mass of metal must detract from the tone or volume a bit.It sounds good though...
I am tempted to put in a thinner bridge plate, and a new standard bridge and saddle, just to see what it would sound like. I am not sure at this point, whether this is considered a "vintage" guitar and whether this would affect its value.
I did intend originally to get the big body version but I didn't want to wait a month! But I really like the sweet tone of the smaller body.