More help at the chest = opportunity to create more speed = easier lockout. Trust me, that slingshot is still helping thru the top. Now if you wanna do, say, your full range 1RM for high reps with a board and a slingshot, go for it. But my experience over 12-15 people I have trained with is that light accommodating resistance creates better speed off the chest. Stupid heavy accommodating resistance will get your lockout tuned up. Like a close grip board press in a slingshot.
Trust me, I've benched in competition with five different grips, I've benched in every kind of slingshot/ram/bench daddy, of all strengths, and like 5 different shirts. Lockout in a slingshot is 100% technical just like it is in a shirt.