I owned an iPad 3 so I know what you're talking about in terms of forget-ability. Apple really abandoned ship quickly with the iPad 3 as it was updated to the 4 quickly and it just never performed to its potential.
I have noticed the lack of RAM on my 6+ a lot. I am a heavy user, and I'd say 3-4 times a day I have an app crash, Safari freezing, etc. This is something I never experienced with my 6 which I had since release day (same apps used as well).
I suppose as more native content to the 6+ becomes available from web and iOS developers alike, it will become a more smooth experience. I believe some of the software Apple is clearly using to downscale to the 1080p screen has some obvious flaws, however I don't think it's something they couldn't fix with a significant update.
While not all of these issues would be fixed with 2GB RAM, there are definitely some freezes that are caused due to the rather lacking amount of RAM on the 6+. For example, when my audio (either Bluetooth or headphones) cuts out when my 6+ freezes, that's a classic tell-tale sign of insufficient RAM. I imagine Apple chose not to include 2GB of RAM not because of cost but rather they wanted to avoid fragmentation in their product line (Apple hates fragmentation). They decided to go with an efficient scaling system that would compensate for the necessary RAM, however they forgot to make the system efficient.
I imagine that the next major iOS update will make the 6+ nearly identical to the 6 in terms of freezing and crashing with some software tweaks and enhancements.