I also just downgraded to 5.1.1 on my 4th gen, and the situation is similar.
-Battery life is bad, but that's due to the CPU never shutting down
-RAM is an issue when a lot of apps remain open in the background, and some apps do refresh themselves sometimes
-iMessages work fine
-Apps-Fine.I have older versions
I guess the iPod Touch 4g really is better then the 3g, especially due to the A4
I'm surprised that iMessage works for you. I thought maybe the problem was caused by my jailbreak, so I restored it but it still doesn't work. Disappointing.
I've been wanting to pick up a 4th gen to add to my collection, I'm just waiting for a 64 GB to hit the refurbished store. Currently only 32 is available.