Going from an 808 front and rear to an 808 front and 900 disc rear will net you about 10 seconds according to Zipp in a 40k tt.

In addition according to Zipp the 101 is a bit slower than an 808 at low yaw but comparable to an 808 at higher yaw and even a little faster way out. So going from a 101 to a disc would probably get you a little more than the ten seconds quoted in Zipps data.