Well my problem with Vista and the beta DWA-552 (connected to a DIR-655) drivers revolves not around performance of the card but performance of the OS.
After installing the DWA-552 and the beta drivers, for some reason, every 20-30 seconds my computer would get choppy. My mouse would have a noticeable lag; closing windows wouldn't be a smooth as normal; and, worst of all, music playing in Winamp would be interrupted every half second.
I uninstalled Vista and went back to XP Pro, hoping it wasn't the card that was the problem. And true enough XP Pro doesn't have the same problem.
I really have no choice but to wait until the Vista drivers get out of beta before I can install this card back on Vista again, which is a shame since I prefer Vista as an OS more than I prefer XP.