Are you measuring the amount of watts you’re churning, yet?
If so, you can figure out if that 15,000 BTU AC is maxed out already or not.
Total wattage consumed in almost all cases for IT equipment translates almost 100% to heat produced. So converting BTUs to watts, you can compare watts consumed against your AC’s rating and as long as the AC “watt capacity” is higher, you’re winning the cooling game.
In your case, 15,000 BTU is equivalent to 4395 watts (by multiplying the BTU rating by 0.293). If you already have four 1200 watt power supplies feeding your rigs and they’re drawing near their peak, you’ve almost definitely maxed out your AC unit already.
You’re also losing a good deal of efficiency through cooling the whole room, albeit you’re also getting “help” from central HVAC as well. It’s a total volume game and it’s easier to cool smaller volumes of air than larger.
Which means, counterintuitively to the whole “open air” design of these rigs, enclosing the rigs in a rack and channelling the AC 100% into the enclosure with a fan at top pulling heated air out and channeling that out of the room will avoid “thermal waste” of cooling whole room and at least match capacity of AC to the heat generated by the rigs much more efficiently.