Performance enhancing proxies have existed (and have been deployed) for a long time. You will for example not find many, if any, satellite data providers withou

Performance enhancing proxies have existed (and have been deployed) for a long time. You will for example not find many, if any, satellite data providers without a PEP somewhere in the mix. Nor will you find many mobile operators who don't have a transparent HTTP proxy, even if not a generic TCP one. The implications of PEPs are discussed at length in RFC3135, and not really discouraged unless they interfere with the end to end principle on the application layer. For your specific example, using the ACK stream as a liveness signal isn't in general very useful. For example the kernel on the phone will happily continue ACKing for a long time even if the application that's consuming the data becomes wedged somehow and can't process it.