[srj]

As this requires reassembling the HTTP request to add the additional header, this probably introduces extra latency too.

Fortunately https is becoming more pervasive which bypasses this and any other transparent proxies.

[tedd4u]

The carriers are working to subvert this -- see the IETF draft for "HTTP/2.0 Explicit Trusted Proxy" or read this article: http://www.theregister.co.uk/2014/02/25/evil_or_benign_trust...