I have considered adding a roller type, but after a whole lot of sailing, I don't think it would make any difference. The Morgan is not a high performance boat. Start with getting the rig tuned properly, with the mast plumb and no or minimal bend. As is, the issue could be either the backstay, or the forestay is too loose. Fix that, but don't just assume the backstay is the culprit. Then get new sails, and talk to the sailmaker about bend. The amount of mast bend required, if any, is built into the sail. With good rig tune, and new sails, you should note a real difference performance. Adding an adjustable backstay is a specialty trim, that won't likely add any gain.
An interesting note on mast bend, that I have only recently figured out. When I left SF in 2018 in the Pacific Cup, my boat was set up with a fair amount of mast bend. I don't think it helped did anything noticeably. However, I have always had an issue with the mast "pumping" while at the dock with a mild cross wind. I would get as much as 4" of fore/aft movement at the spreader level. It was quite violent, even with only 10 kts of wind. Overtightening the lowers made no difference, nor did anything I tried. It became quite frustrating, and I even rigged a babystay to try and control it, which helped only a little bit.
Well, this past year I reset my rig tune, plumb with no bend. The pumping stopped. I have several ideas why. But I now really feel it important that the mast on our boats not have excessive bend, if any at all, and that the mast be plumb, without any rake.