Why My android device show less RAM and Slow at Jelly bean?
I'm also using the same  device. Also upgraded to jelly bean.
Well, to your above queries, just remember that higher RAM usage in jelly bean is actually supposed to "utilize" the device properly. Logically, we buy device with high RAM for what? So that we get faster performance. But there is always tradeoff between performance and features. Jelly bean offers more features but takes more RAM.

Regarding rooting the phone. It  is always a grey area. while rooting voids device warranty, it also enables user to  control almost every aspect of the device. But do it if you are willing to invest time understanding the intricacies of rooting.

cheers!