Sorry, but I'll swing at this one. How are they bad in the starting application?
The Toyotas are not a multistage golden calf charging system like you'd find in a closed loop DC/DC charger so implying they have any "charge profile" beyond a basic temp curve is disingenuous and the temp comp is the probably the normal 30mV/degC for FLA (AGM is something around 24mV/degC dV/dT). Where it gets the temperature measurement is unknown but the battery is not instrumented so it is a derived measurement from alternator regulator temperature and probably deviates from reality in cold weather where the regulator warms faster than the battery does. Outside of the temp comp control it is a constant voltage field controller with no shunt so it has no idea of the battery status of state of charge (no float reduction, just hammer it at bulk charge voltage with whatever the cells will eat)
I agree that on paper the underhood temps reduce service life but in my experience running AGM underhood I have not seen it be less than the normal LA battery life in hot climates. I get 3 or maybe 4 years out of the batteries no matter their flavor (and FLAs I don't top off the electrolyte so they get equal abuse from off gassing and low fluid). lead acid is a 600 cycle chemistry so it is a race condition between cycles and thermal degradation. Anything above 95F starts causing unrecoverable cannibalistic SO4 redox chemistry in FLA and AGM. AGM does charge better below freezing than FLA and in pretty much all conditions charge faster than FLA (assuming the 130A? alternator can drive the load - I don't see the charge voltage falling off so it is carrying the load).
The internal resistance is lower on AGM so not grossly low resistance like a dead short that will make an alternator cry (this is hard to quantify without having two batteries of nearly equal capacity, side by side with known charge levels and temperatures). The charge time factors reflect the loading differences (the chemistry is the same so you are looking at the glass mat factor vs resistance of ~1mm? of acid plus the same Pb and PbO2 plates and how much power you can pull in).
I didn't find good data sheets from interstate and the H6 so I used the Rolls manual to compare their AGM to FLA charge voltages in the attached.
Long story short- the AGM will be fine for a rig - Draw it way down and you'll hammer your alternator diodes and regulator when you fire up your rig. Similarly you'll also hammer them if you draw down and flatten a stock FLA/SLA battery or if you hang a 50A DC/DC charger and AUX batt in addition to the OEM circuits, 19 iPad chargers, starlink, 900W HF radio for calling in the Dominos order in Ulaanbaatar from British Columbia, and the required overland margarita mixer. Yeah, the alternator temp curve could be "better" but it is fine (cats and dogs won't be sleeping together because you install an AGM). The highest voltage I've seen on a 5.7 tundra or 200 is 14.8V. You don't need to use an external charger to float the battery constantly as a starting battery application. Sure if I was using it in an offgrid primary power application I'd care about every coulomb of possible capacity and extended life but in 99.9% of automotive applications the battery draw down during normal use is maybe 15% at start up then covered by alternator for the short charging duration. You may be the outlier but if your power system is high demand you have shunts installed and closed loop charge controllers to monitor SOC.
In the spirit of experimentation vs opinion I'll run out and get the post start charging amperage and voltage on my rig. She's been in a 13 deg F snow drift for almost two weeks so the battery should be low'ish and should see near peak alternator output for a short period.
How cold was it? The hood was frozen shut and I pried it open with an ice scraper...
Pre start battery at 12.4v and -0.2A draw from just doors open and ECUs coming on after opening the door
EFI and fuel pump priming 12.06V and -4.7A
Start up and running after 30 seconds 14.21V and 14.5A (the 200w held steady at idle and with a 2000 rpm throttle bump)
Volt/Amp steady for one minute then I turned it off because it was freezing outside. Measurements made with Flukes I trust and have cal'd. Not sure what the temp curve is for this regulator but that is a low'ish output voltage for sub freezing by any interpretation of temp comping FLA.
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