I'm running this on an iPad 4 and while it's definitely still playable, it's definitely not the smoothest experience. The whole game is relatively choppy in many aspects from zooming in and out, selecting various things, etc. It's a ton of fun and I can't put it down, but the optimization is definitely questionable thus far. I've got 40 people exploring at the moment and everytime I cycle through to check how many stimpacks they each have remaining, it's a painfully choppy process and takes about 20 seconds to initially load so I can start cycling through them.
http://www.gamesas.com/topic/1522276-fallout-shelter-things-to-know/http://www.gamesas.com/topic/1522276-fallout-shelter-things-to-know/
It's right there in the sticky in this exact forum. There will be no "better support" for devices with 512MB, because devices with 512MB are flat out not supported at all in the first place. Go read that.
Since they're in cahoots with Apple, it seems like they're leading people to upgrade their devices. Clever.