I have used Denicool silica crystals, which is basically commercial-grade silica, and it changes the flavor of the smoke for the worse in my opinion. I started using these when I was smoking a lot of 5100 Red Cake, and noticed it right away. It strips the sweetness right out of the smoke until you get to the bottom and heat them up. There's all of the missing flavor. If you want to source your own crystals, just know that there are two commonly-found types of commercial-grade silica crystals, one is "safe", one is not. I forget the terms that differentiate the two, but it has to do with whether or not the pieces will shed dangerous particulates.
I like to use meerchaum chips in the bottom. I find that this changes the character of the smoke for the better, while not really changing the flavor. They also hoist the tobacco up above the draught-hole, if you are having problems with that. (I have a meer-lined bowl with a high draught-hole.) It can also help when loading smaller pieces of tobacco, acting as a filter to keep the tobacco out of a draught hole. Plus the chips "store" a small bit of the overall flavor, which is nice at the bottom of the bowl when you heat them up a little. I have no idea if these shed dangerous particulates or not, but I just make sure the pieces aren't dusty when I load them in the bottom.
I have not used the keystone pellets, but I believe that they are expanded clay pellets. I wouldn't mind trying them, but the meer chips seem to make more sense for pipers who want to put something down there for whatever reason.