The performance impact is marginal for most tasks. The only difference will be gaming, and as eagle commented, a simple overclock can resolve the problem with little effort.
Now to my opinion.... I would buy the retail card, directly from the Matrox online store. This way I know the most support dollars goes back to Matrox itself, thus insuring the continued existence of Parhelia driver development and hardware support.
Matrox is not exactly your typical video card manufacturer, as it is a privately held company, thus I want to show my support in the best way possible: money.
Robert Richmond