can i go to 5grns in .38 brass and be OK?
You could work up slowly to that in your gun looking for pressure signs along the way.
But I wouldn't. Titegroup is a fast powder and too unpredictable for me to try outside published ranges. I'd load light .357 and swipe a magic marker across the primer to tell them apart.
I'd rather have a few light .357's around that wouldn't hurt ANY .357 mag gun than a few .38's that could tear something up.