You're slightly missing the point. It's not hampering CF/SLI at all and I've already agreed that they're running at the maximum as per spec but the spec itself is the problem and doesn't represent that good value for money IMO especially for an SLI option.
By pairing 2 mid range GPUs with a 192 bit bus you're just exacerbating the problem when the cards are running individually (i.e. they're bottlenecked by their memory bus). You're adding another load of GPU power but the bus width is obviously the same and becomes an even larger limiting factor.
+1
It isn't that it is a bad idea, it just isn't a wise buy from the off.