There are good VSTi's, and there are bad ones. Just as there are good arranger drums and not so good.

Compare the S950 with the best... Try BFD3 http://www.fxpansion.com/index.php?page=205 or Superior Drummer2 http://www.toontrack.com/products.asp?item=30

If you think those don't compare more favorably than the S950's drums, this might account for why your mixes tend to bury them! You have to get your drums up to the point where they can be heard well before it makes any difference...