If you are sticking with a given powder for a caliber I would say there is no benefit of lowering powder charge for pistol or SBR loads, all you would be doing is hurting the performance in a barrel length that is already hurting for performance. Additionally when you decrease a powder charge you start to get inconsistent ignition which will effect accuracy and velocity from shot to shot. It won't make that specific load unusable in a longer barreled rifle but it will certainly give it undesirable characteristics in any rifle / barrel length used.
You could switch to a faster powder for shorter barrels. Slow powders for a specific caliber thrive in longer barrels (and often with heavier bullets, but that is a different conversation) but often give up some of that performance in shorter configurations. The other advantage to switching to a faster powder is economy. If you do use a faster powder for a short barrel you will lose a tiny bit of performance in the longer barrels but if you able to make an accurate load who really cares. A piece of paper, steel going or even a game animal won't be able to notice the slight drop in velocity.
I hope this helps in some way, good luck to you with your load development.