You will have No problems killing deer with 40# just take high percentage shots well within your effective range.

IMO I would use a 100 grain head to improve FOC. I am not a fan of the lighter broadheads. If you want to cut overall arrow weight use a lighter, properly spined arrow and put some weight on the business end to keep the FOC at a minimum of 12% for broadheads.

Preferably more!

Dan