Disagree about this 3G baselining, IIRC you can't play multiplayer games over a 3G connection. You probably won't be able to use remote play over 3G anyway.
The video for the Vita will be downscaled to 540p, then compressed by the PS4 (presumably, no reason to stream 720p when the Vita can't display it). Even so, 3G speeds wouldn't be good enough, unless the video streaming interface is smart enough to dump bitrate and maybe even lower the resolution, but then even if it would work you might not want to play it.
Input lag won't have much to do with data throughput, controller data is pretty simple so it will be related more to ping times, which are also horrible over 3G but saturating the throughput doesn't increase ping times.

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