[Butcher]

Silver's thermal conductivity is 419 W/m-K when pure, sterling is slightly lower at 410 W/m-K, but still higher than copper - C110 is 388 W/m-K. That's not 1/3 higher by any means - you'd have to be looking at diamond to beat copper by that much.

However, if you're going to make a water block out of silver, you should go the whole hog and use pure - it's going to cost loads more so you'd want it as good as you could get it.