I've not used the Perception 820, so I can't comment on it. However, I have used two other mics from the Perception line and they're a considerable step down from the rest of the AKG product line. They're stamped out in a factory in Shanghai and made as cheaply as those clever Chinese engineers can deliver. Don't ever drop one, lest internal components become detached and start rattling around inside the body.
If you want to invest in one really good microphone that'll handle almost any scenario and do justice to a top-notch singer, figure on spending a grand or more. Go for a Shure KSM-44, AKG C414 or Audio-Technica AT4050, all of which are extremely versatile and well-built microphones. Of these, I believe the AT is the least-expensive, the Shure is the quietest and most rugged, and the AKG offers the greatest versatility.
All of those have less-expensive cardiod-only versions (KSM-32, C214, AT4033) that sound just as good but lack some features that an all-around microphone should have.