Backstay tension is something that only really affects fractional rig boats.
Not strictly true. As it applies to the main: Yes. But backstay tension affects the forestay tension on a masthead-rigged boat, allowing you to adjust the headsail's depth. (We have a backstay tensioner on our masthead-rigged boat.)
However, it does help flatten the mainsail and headsail considerably.
It's my understanding backstay tension has little-to-no effect on the main on a masthead rig.