It would obviously send the data out twice; but once would be over the cell network and the other time over WiFi. Doing so would also help with the far more common Cell<>WiFi transition cases. However as the phone still only has once Cell radio set it would not help with transition between communication bands or carriers.
Since the accounting that actually matters (to Google) is at the data center and based on the way that peering agreements work, I suspect Google would actually find /incoming/ data to be either unimportant or even a benefit to their overall cost operations based on bringing their usage closer to equal. The consumer would only normally care about the cell min used; unless their local ISPs have zero competition (see 90%+ of America).
Since the accounting that actually matters (to Google) is at the data center and based on the way that peering agreements work, I suspect Google would actually find /incoming/ data to be either unimportant or even a benefit to their overall cost operations based on bringing their usage closer to equal. The consumer would only normally care about the cell min used; unless their local ISPs have zero competition (see 90%+ of America).