It would probably be easier to stream over 3G/4G but the data consumption would likely be between 500MB-1GB per hour.
There are mobile tariffs allowing unlimited data, such as Three's One Plan for £20 a month, but mobile streaming via Sky Go is usually 1-3 minutes behind, and if the coverage drops then you'll also lose the picture and usually find it picks up from where it stopped - so will now be even further behind (this might be better than just returning to being live, or not, depending on what you're watching).
Only yesterday I was out and getting texts informing me of how bad Spurs were doing, while then having to wait a minute or two to see the next goal on Sky Go!
I'd say that it would make more sense for a certain amount of material to be downloaded and displayed, much how I expect it's done on things like the Heathrow Express, periodically. You'd then have some rolling news, sport and so forth - but nothing actually live. (BTW, how much would the coach company have to pay for broadcasting Sky, especially sport, to passengers - rather a lot more than a home viewer pays!).
But with so many people having their own device, which gives them the choice to watch exactly what THEY want, I can't see onboard systems being worth the investment unless they're done as loss leaders to get more people to travel over a rival service that is without.
I'd rather have a seat back screen and a USB slot that will charge my device (on Virgin planes, they don't charge - at least not in economy) and let me watch my own content on the bigger screen. That seems to be the way a lot of in flight systems are going, as well as many hotels that have ports to let you hook up your phone/tablet.