The optimal iOS (in my biased opinion) to ever run on any iOS device would be iOS 6 for devices capable of running iOS 6, if not, then iOS 5 as they have low resource demands, and is flawless with respect to jailbreaking. (Just putting it out there)
But with respect to iOS 8.1, as scifan57 said, it's relatively stable at it's current state when compared to the GM release. Jailbreaking it isn't difficult and yields minimal issues and has the same risk as every other previous jailbreak (which is basically none). As for performance, the A5X processor in the iPad 2 runs iOS 8.1 remarkably well unless you're overloading the device with RAM intensive apps such as games that requiring caching as they don't save at immediate intervals and has high levels of rendering simultaneously (i.e. The Walking Dead or A Wolf Among Us), but it's rare to experience crashes/lags outside of that as well as other notoriously demanding apps.
iOS does a pretty find job of managing resources, so performance-wise, it's always at an optimal level with respect to demand from apps, so as long as the device is still being supported by the iOS, that would mean it's capable of running the iOS at benchmarked levels, otherwise Apple would give the device legacy status. If the device is skimming the surface of the benchmark, Apple would give an adulterated version of iOS for that device such as iOS 7 for the iPhone 4 and iOS 8 for the iPhone 4S. AFAIK, the iPad 2 is running an unadulterated version of iOS 8, meaning there aren't any software means of compensating for lower hardware specs, which would equate to the iPad 2 having a pretty good score in benchmarking compared to the iPhone 4S, which is to be expected due to the screen size difference and the hardware requirement to keep up with the screen size.
TL;DR: iPad 2 running iOS 8.1 performs pretty well taking into account its specs.
