[junglejuice]

I've had issues in the past with thermostats, particularly aftermarket units, I'm running my original t/stat now which is at least 12yrs old as that's how long I've owned it.
As you're in warmer temps now try taking the t/stat out and give it a run on the highway and see how it goes temp wise.
Also try and verify if the temp guage is working correctly and remember on a pre-update the temp sender for the guage is at the rear of the head, try measuring the temp with a probe and see if it roughly correlates with the guage.
As for the GC clutches, that's not a brand but a Grand Cherokee clutch, ie from a local Zg or a US sourced Zj unit.
As mentioned I think above lean mixtures will result in higher temps so try switch to petrol and see if the temps are similar under similar conditions.

On the highway the fan/s don't really do a lot assuming that there is no/little actual obstruction to airflow including through the condensor etc (not just things light driving lights etc).

Fancy coolant is NOT required for these engines, even on straight water or elcheapo pre-mixed coolant it should not give overheating issues if the "system" is "effective"...

Lastly, mods AREN'T a fix just a band aid/disguise....