140 fps should be okay to kill a deer, but if you haven't already, you can switch to a fast flight string and you'll pick up some fps. I switched from D-50 to fast flight on 3 of my bows and picked up between 8 and 10 fps.
Hickory should be able to handle the stiffer string, but I never tried it to confirm for sure.
The extra fps probably won't do much for penetration but it could make you feel more confident that your equipment can take down what you're hunting. And that's worth it to me.