I think Britt has more intrinsic overal talent than either O'C or Kaczur. He can develop into a very good starting LOT, and that is a precious commodity.
It may even be hurting him. While Kaczur played LOT as a rookie in an emegency, and did OK, I think the coaches felt that he did not have much upside at that position. O'C is a natural ROT. He has the size and toughness to play that position; but that is the ONLY position taht he can play.
Kaczur may be a better Guard than either, Britt or O'C and a good but not exceptional tackle, except that the Pats have lots of good Guards.
It is a pleasant situation to be in. How much Britt is being held back because they need a spare LOT, is hard to measure. Bill has done that with other reserves who could be starters. Think Jarvis Green for example.
Britt is still young, but i see him following that career path. There is a difference however. Light is a good but not great LOT. OTOH, Seymour is the best Defensive Lineman in the league. Britt might eventually pass Light by and start; whereas Jarvis has a much tougher row to hoe.