We recently published a paper on this exact issue and quantify the degree to which AV / corporate middlebox systems degrade the security of HTTPS connections. The tl;dr is that we find an alarming amount of MITM on the public internet (5-10%), mostly due to AV/middleboxes, and they almost always degrade the security of the connection.

*https://jhalderm.com/pub/papers/interception-ndss17.pdf