I'm not sure you'd get ANY extra clearance. The limiter is almost certainly the chainstays, and on newfangled carbon bikes they don't necessarily get wider as you move toward the rear. Remember we are talking about moving the rim less than an inch rearward.

If swapping to 650B got you into that 1.8-2.0 tire realm then I'd say, go for it. But I'm just not seeing it.