By the way, the goal was to stop charging the battery, therefore battery degradation can be reduced. "Keeping battery level around 50%" is not the goal itself. in other words: leave it to natually self-discharge, instead of a brutal drain/recharge loop... Anyway, this gual doesn't seem to be...
According to my experience, this is not true. The CPU frequency cannot raise above base frequency (turbo boost) any more, if AC adaptor is unplugged, even if the power plan is switched to "high performance".
IMHO, this is a bad idea. If you simply disconnect the AC adaptor, the performance will immediately turn degraded, since the battery cannot provide as much power as the AC adaptor.