Before changing your light I would try to straighten the run of your ductwork as much as you can, and make sure that your passive intake is at LEAST as large as your exhaust; preferably a bit larger. What does your intake look like? Is it big enough, and is it low in the room?
Another think to think about is CO2. Enriching your atmosphere and running a silica supplement will allow you to run safely at temps in the high 80s with even spikes into the low 90s. However, if your humidity is quite low, you may find your plants drying out too fast.
Try to solve the heat problem with ducting first, including making sure that there are no leaks in the run from the portable a/c to its exhaust point. One thing to consider is actually moving the AC outside the room too, and setting it up to cool the air at the intake point. Something like a small tent around where your intake is, that contains the a/c and CO2 if you have it, allowing for heat generated by the a/c unit itself to escape (yup! pumps make heat!), is one approach. I run my a/c in the 'lung' area right outside my flower chamber and it kicks on at temps over 75'F. This way I am assured that air ENTERING the room never does so at high temps, and has some wiggle room to get warmer before it is exhausted.
There are as many solutions to growroom problems as there are growers. Don't be afraid to get creative. Air flows easiest in straight lines, confined to smooth ducting. Heat rises. Moist air carries a great thermal load, and exhausting it smoothly is KEY. Plants tolerate higher temps when they have ample CO2 to the leaves, O2 to the roots, and plenty of water available as long as it does not block the O2 in the root zone. Within these basic rules, you can probably arrange something that works!