I'm currently shooting an arrow weight, with a 100gr broadhead, at 370gr. I was wondering if I dropped to an 85gr tip, dropping arrow weight to 355gr, would I have any problems with penetration on whitetail deer? My bow is set at 70lbs.

I doubt that it would but it could screw up your foc and change your arrow spine as well as the bows tuning.

You will not loose 3 lb of KE IF you do not need to tie a parachute( 4-5” helical fletch) on to stabilize it. The bow must be tuned properly; a light arrow traveling sideways will come to a standstill fast.

A light arrow will need a cut on impact broadhead, & it needs to be full length for leverage, no overdraw, to help prevent flipping.

Some hybrid mech broadheads are a good bet here.

The KE you have would penetrate elk if hit in the kill zone & that is the only place to hit??..