If you have your GPUs only running the fans at 30-40% or so and your temps are still only in the 70s Celsius (going by the image you linked) then you really are doing great. If you would run your fans at 60-65% you would have lower GPU temps and still have plenty of longevity for the fans themselves.
I think the only thing you really need to be worrying about is ditching that AC unit and getting some good industrial exhaust fans installed... that will save you allot each month on your electric bill and keep things just as cool and probably cooler.