I managed 14/28 with 46/30t on a 33% hill with moderate weight on the bike, I reckon 32t with XT would be enough for you, I currently have a Tiagra 10spd long cage rear derailleur on the bike so that was just enough for the 46/30 chainring.
People ridden with 48/28t chainset before, so it shouldn't be a problem, sheldon ran the same set-up for light touring with a better range than a 46/30t;
I managed 14/28 with 46/30t on a 33% hill with moderate weight on the bike, I reckon 32t with XT would be enough for you, I currently have a Tiagra 10spd long cage rear derailleur on the bike so that was just enough for the 46/30 chainring.
People ridden with 48/28t chainset before, so it shouldn't be a problem, sheldon ran the same set-up for light touring with a better range than a 46/30t;
edit: it's axctually 50/28t