Ginseng is a generic term referring to biological compounds belonging to the genus (or class) Panax. Chinese ginseng, for example, is a member of the genus Panax. Siberian ginseng, on the other hand, is not a member of Panax. Rather, it is classified as Eleutherococcus senticosus and is distinct from such varieties as American, Chinese, Korean and Japanese ginseng.
Unfortunately, Siberian ginseng has not been widely studied in western medicine, including its effect on athletic performance. To my knowledge, three studies have been conducted with Panax ginseng, the most recent and well-designed of which was published in the Journal of the American Dietetic Association in 1997. This study showed no effects on any physiologic or psychological parameters of performance including VO2 max, minute ventilation, heart rate or perceived exertion. Bottom line: No improvement in performance.
Despite the relative lack of research on the effects of Siberian ginseng, it is important to point out that there have been case reports of potentially harmful effects of ginseng supplementation including hypertension and interference with medications (mainly Coumadin) as well as side effects such as sleeplessness and diarrhea.
Finally, let's clarify that ginseng of any type is technically considered a dietary supplement so it's manufacture and production is not regulated by the FDA. That being said, there has been concern in the past regarding purity, not only of ginseng, but of many dietary supplements. In short, you cannot be entirely sure of what you're consuming when it comes to dietary supplements.