Magnepan 1.7i vs 2.7i?

I've had 1.6, 1.7 and  3.7. As a rule of thumb, bigger is better with Maggies.

While I think the true ribbon is WONDERFUL, I'm not sure it's worth the upgrade from the 1.7. If you were going from the 1.6, then yes.

The 1.7 tweeter(s) are pretty good.

Hell yeah!!!  I'd move up to the 2.7i speakers in a heartbeat!!!  A few years ago, I owned and loved a pair of .7, then 1.7i speakers, but I always longed for a pair of the larger 3.7i speakers, but I just couldn't swing it because I just didn't have space for them.  At that time, the 2.7i hadn't been released yet, but they would've been too large for my space as well.  Yes, if I had to space, I'd definitely go for the 2.7i, or better yet, the marvelous 3.7i speakers.  Happy listening.  

As a person who is fussy about treble, I would go for the 2.7i myself.

"If the shoe fits, wear it." In this case the shoes are Magnepans and the foot they have to fit is your room. More than any direct radiator except big horns, Magnepans are are best chosen for, or maybe fitted to, the room size they are used in. it is simply in the nature of flat panel dipoles needing the breathing room to properly work their magic. I recently bought and quickly sold a pair of ,7s when it became clear I did not have a Magnepan-friendly space for them. I am no Magnepan hater, far from it, having owned several pair since as far back as 1976. 

The current lineup of Magnepans goes from 13" to 29" in width and 48" to 79" in height - variations of over 2X in width and 1.6X in height. These things matter more  in dipoles and line source speakers far more than in traditional box speakers because they determine in large part how the speaker acoustically loads into the space. the 2.7i is something like 25% larger overall than the 1.7i, so if you think your room can accommodate them, then do it if you wish. If the 1.7s are a good fit for your room, then I probably wouldn't make the move.