My own personal opinion is that limiting tethering will do absolutely nothing to ease congestion and improve slow speeds experienced in some areas. If you look at the forums of networks which never allowed Tethering to begin with, they are full of complaints. Giff Gaff, as one example that I know of, had a huge thread some months ago, regarding slow speeds, and at that time, non of its data+call tariffs allowed any kind of tethering. EE are the same, their tariffs, I believe, have tethering which is limited to the total monthly allowance on the tariff, but once again, complaints about speeds are again evident from some customers, some of these complaints are even in 4G areas.
So based on this, I think that those thinking that its going to be a magic solution for any local speed problems could end up disappointed. The only thing which is going to prove the point either way is time, but I suspect by the end of Q1 2015, people will be having exactly the same issues as they have now, nothing will change, other than the monthly fee that a lot are now paying for their 'revised' tariffs.