First, let me say that I don't recommend that anyone exceed the manufacturer's recommended max loads as you do at your own risk.I would be curious what your "full charge" means, because 1590 fps with a full charge of power pistol to me indicates going well over book. Alliant shows 10grs as max for 10mm 165gr Gold Dot @ 1314 fps, and I personally got almost exactly that out of a G20. An extra 1.5" isn't going to give you 275 more FPS unless you're running way over, which I get, I do it too sometimes. In addition, Power Pistol is very good and I find it to be very consistent, but it's not the powder you want to use for absolute max, there are better options.
My fastest 165gr 10mm is using 800x and pushes 1400 fps from a G20 and that's hot, your load (using Power Pistol no less which has much less potential than 800x) is nearly 200 fps faster? You might have got some bad readings.
Also, the .45 Super will flat out whip up on the 10mm, it simply holds more powder and can be loaded identically to the 460 Rowland, but does need a compensated barrel when you move past 10mm territory in order to slow down slide speed.
My hottest 10mm 180gr from a 5" KKM and hottest 45 Super 185gr 5" KKM:
10mm 180gr: 1,387 fps avg (769 ft-lbs)
.45 Super 185gr: 1,570 fps avg (1,012 ft-lbs).
BTW, 200gr XTP from same 5" KKM .45 Super was 1475 fps, so your 1300 fps with 6" apparently isn't nearly as spicy as your 165gr 10mm load, which has to be like 50K psi or something is off.
For heavier stuff, my fastest 200gr WFNGC 10mm was 1315 fps from 5" KKM (800X) and for 5" KKM 45 Super, a 255gr WFNGC at 1320 fps. Same velocity really but over 25% more bullet weight not to mention bigger hole in target (767 ft-lbs vs. 986 ft-lbs)
I'm not dogging 10mm at all nor am I trying to be argumentative, it's just that when you say the .45 Super can't touch the 10mm, that's not true.
My 165grn load in my neighbor's stock Glock 20 at the same range and at the same time, clocked almost 200fps slower so as I've said many times before, the extra 1.6" of barrel length produces a significant increase in velocity at 6,100ft asl on an 84° day.
What you shoot, where you shoot, and what you load may be different. I've tried a few 800x loads and all it did was produce a huge flash, a loud bang, and nothing spectacular for velocity or accuracy. Maybe it's better in the standard length, 4.6" barrel, but in my 6" one, Power Pistol produces better results for me.
It's true that 10.0grn is max load per Alliant, however, these loads I use for 200yd steel plate shooting and I do go over a tad to 10.3grns. Pressure signs on the primers was surprisingly not as extreme as I had assumed. As to the 45 Super, I have checked load data from many sources and worked up 10.0, 10.2, 10.3, and stopped at 10.4grns of Power Pistol with 200grn GDs as the pressure signs on the primers were getting up there.
As I said above, neither pistol caliber uses a muzzle brake as I'm not a fan, however, I do use a 24# recoil spring and the 4¾ oz increased weight of my slide and barrel over a stock G20's combined work well to tame muzzle flip and the recoil impulse.
So, it looks like the data you have for your 4.6" barreled G20 produces loads that perform differently at your elevation and humidity level with different powders than my 6" barreled G20L does with Power Pistol at likely a higher altitude in a very dry climate. Go figure.