Sensicore adds a little bit of weight to the shaft, but not much. It is essentially a shock absorber inside the shaft to reduce the harsh feel and vibration on off-center hits. I believe the insert is "tuned" to allow feedback, just not the "OW!" type of feedback.
I used to have a set of Pings with the Cushin insert in the shafts, which is very similar to Sensicore. I didn't think they made a big difference either way, but that was with a very forgiving set of irons. With the 690.MBs, you might encounter the stings more often and find Sensicore useful.
If at all possible, find a golf shop that has demos with Sensicore-equipped shafts so you can feel the difference yourself.