Tommy I had a decibal meter with me when I tried the e-sax. It was on an iphone but it is reputed to be to reasonably accurate.
The back ground noise in the shop was about about 60db - I played the saxophone without the mute and it was at around 90-100db (including background noise). Playing with the sax inside the mute it was at about 70db. So the mute cut out about 25db.
Apparently 'a quiet bedroom' (quoting the decibel meter manual) is 40db and an open office is about 65db.
I'm not sure what conclusion can be drawn from this as sound is measure on a logarithmic scale but at 70 db it would be like two people holding a conversation at 1m apart.
I hypothesie

therefore that if you were somewhere adjacent (e.g. in another room in your house) in a totally quiet room and someone in the room where you normally practise was holding a conversation then would you be able to hear them? If not then the mute would give total sound proofing.
I'd suggest that in the average house the level of sound would be reduced noticeably but it would still be audible unless the person in the other room had some back ground noise (e.g. television). However for neighbours, though an adjoining semi detached wall, it would probably reduce the sound to close to inaudible.
Steve