Posted this somewhere else but figured you guys might also find it interesting
First off, it became apparent that there is a pretty big difference between the perk being the main ability or a sub ability on your gear. Having the attack perk as the main ability on my gear raised my damage by about 9-10%, while having it as a sub ability only raised it by 3%.
When trying to stack these on top of each other, I also noticed there was a diminish in how much each stacked perk would grant you. For example, 2 stacked main attack perks would grant me about a 16% damage increase as opposed to 18%, and 2 stacked sub attack perks granted me about 5.5% damage increase. This shows the importance of your main abilities, as not even 3 stacked sub perks would equal out to 1 main perk of the same type.
I did a little more testing to see some optimal setups and found these interesting:
1 Main / 2 Sub attack on the Gal shooters made them go from a 3 shot kill to a very nice 2 shot kill
2 Main / 1 Sub attack on the Rapid Blaster made it go from a 2 shot kill to a nuts 1 shot kill if you get a direct shot