Boots of Speed can tack another down-2 on there in open ground.
And where are you seeing the bolded, that a Cloak of Displacement gives 6 AC points? My handy DMG tells me the Cloak only gives 2 points of AC, though the first attack that would hit the wearer always misses.
While this thought experiment is neat, are PC's allowed to go below -10 AC in 1e? (I went looking and while I found more fuel for the forgotten rules thread, I haven't tripped over a specific reference in the 1e rulebooks). I've been trying to remember which issue of Dragon it was where someone asked about being able to hit -11 AC because some dragons can (and go beyond), and the response was "if you lived for thousands of years, you could have an AC like that too". I mean obviously that would have been in the 2e era, but it was the first time I really recalled being cognizant that there was an actual limit. I mean, the rules had surely mentioned it somewhere, but it hadn't really clicked. I think it was the instruction manual for my NES port of
Wizardry that defined AC -10 as being equal to tank armor, and in that game that was a hard limit.
And with Dragon magazine in hand, I didn't need to look up the rules, lol, I knew the secret lore! You couldn't have -11 AC as a player character! But of course, that didn't stop arguments.
I had a friend who truly, madly, deeply, insisted that the Dexterity defensive adjustment was somehow different from armor. "If you wear magic armor, it gets a +1 or higher bonus to your AC", he reasoned. "So a -2 from a 16 Dex is different, so if I have -9 from armor, I can have -11 with 16 Dex."
That's when we all started diving into the books, while he assured us "it's in the rules, I saw it", but we never found that. But what we did find was page 89 of the 2e PHB which said:
But surprisingly, that didn't stop the arguments. "Yeah, it says -10 is the best (very powerful magic armors), it doesn't mention Dex! And also, if -10 is
the best, how can -11 be a thing?"
Eventually, the guy who DM'd the most for us all put his foot down and went with Dragon magazine. And he eventually revealed he did have immortal NPC's who were thousands of years old, and had AC's lower than -10, plus stats above 25 (as he'd started his campaign pre-AD&D when the rules about ability scores were more like they are today). And when Immortal-level play became a thing, he griped about TSR "stealing his ideas", lol.