I personally think iOS4 or iOS5 to be the best of the bunch. My pinnacle on battery life on the platform was iPhone 4 on iOS4. I generally could get 10 hr screen time usage on Wi-Fi and my iPhone 4 once had a month on standby while on airplane mode. After iOS5 with the pull-down notifications, iCloud, more Apple bloatware, and faster GPU of the 4s and later models, battery suffered since with maybe 6-8 hr screen time usage and lower standby times.
My experiences confirm your comments. My daily driver is a 32GB iPhone 4 running iOS 5.1.1. (primarily because of the custom settings I made for it to be fully functional in StraightTalk)
Performance and battery life remain terrific... 2+ years later.
I don't think Apple overtly plans to slow older models down, but they DO make the decision of which older devices are eligible for upgrades. THAT is where they can be a bit more covert about it. But it is a balancing act... too restrictive and they'll be accused of "abandoning" older devices, too permissive and they'll be accused of "sabotaging" older devices to upgrade.
Pre-iOS 7, I think that there was a reasonable case to be made that Apple made it very difficult to remain at an older version... especially with the auto-downloading of the installer of the latest available/supported version. But Apple has with 7.x provided the option to delete the updater.
That's a much appreciated change for me, and one that is making it easier for me to stick with an iPhone.