is 81 degrees an ok temperature?

Hi, basically what the question says. I Know 75 would be a better temperature, but don't want to buy any more supplies for cooling.

is the difference between 75 and 81 degrees large enough to inhibit a significant amount of growth, losing yield?

81 degrees is fine. That's where I usually keep my temps. As long as the radiant heat from your lamp isn't too hot for the tops. I like it between 80-85(ambient) because when the lights go out, it usually drops down to the low 70's, and that's what you want. A 10-15 degree difference between when the lights are on and off. Like I said, just how I prefer it. Hope this helps you. Best of luck!