If you get 4 crappy Rivens, you melt them together and try for a lesser Crappy Riven. So the idea is quite translatable. I personally wouldn't use 4 regular shards -> 1 Tau, rather 2. This would be a better balance since you are only getting a 50% boost. So maybe 2 shards and 50 sentient Cores. (the 50 may be steep but they drop frequently enough in the Hunts to validate it)