I like the pleated back from a fashion perspective, but it does nothing to help with mobility that I've noticed.
I am, measured by my tailor, a 43L. I usually buy my suits in a 44L which causes them to run a bit big (which has been discussed at length here). This way I can play comfortably, however the guitar strap does cause them to bunch up slightly in the front. A contributing factor is that I have broad shoulders, but my chest and waist are small. I have the waists taken in, but there's only so much that can be done with the chest. I would really need MTM to have a suit jacket that fit comfortably and looked perfectly tailored with the guitar strap attached.
As you alluded to, it is the shoulders and arms that are the biggest issue for ability to play comfortably.