I've checked a tuner in U.K. and 20 BHP, 20 lb/ft increase is what they offer. At a guess, you could safely push this higher. I'd also recommend carrying out some simple breathing mods too like adding a high flow air filter, catalytic converter removal and deletion to make the most of any gains.
Go with above advise as it is.
You can only deflate further while you struggle if you have good condition off-road tires (All terrain or M/S). Then you can deflate to 10-12 psi otherwise for road tires (HT tread) 14-15 is ideal.
If you deflate further while having extra load then be careful to avoid sharp turn and go straight up and down than side-ways which will pop-out tire more easily due to extra load under low tire pressure.
Leave it as it. You were struggling because you were carrying a lot more weight. Decreasing pressure is not a good idea. You'll be a lot more prone to a pop out.
Infact back in the day when I would load a 1 ton generator in the back of the bushanab I would deflate to only 20psi but with the weight of the generator the footprint at 20 was that of a 15 deflated tyre. When when on safari with full load of 6pax plus drivee never took it below 15.
Another reason is. Its summer time now and the sand is a lot more softer too.
If you want proper chip tuning with rolling road setup you can try chipcentric in al quoz. Speak to Murat and tell him I sent you.
The other option is go to rashidiya and have a cheap generic map loaded on but there is no rolling road testing. If you're interested I can get you a contact tomorrow.