Probably very effective, but be careful. I tried to use a Webley 8" BBMF silencer on my CP99, and the weight of it gradually bent the UNF adapter to the point that the pellets started clipping. Go for it if your adapter is a nice thick piece of steel, but remember it's still only held in place by one little screw...
An alternative would be to get a Soundbiter mod, apparently they work very well on CO2 guns and weigh next to nothing.