Some quick feedback.
For the sake of clarity, gonna call CB t0, guk tier T1, seb T2
I haven't progressed beyond seb, so i will keep my comments limited to my play experience between t1 and t2.
First, I have to say the progression between tiers is very difficult. The ramp is extremely steep, which is fine and about right imo.
Going from t0 straight to t1 is hard and you ought to wait till you are in your high 40s to solo or 30s to group.
My biggest problem is in t1 to t2 and the big named mobs in each zone from t1 over to t2.
IMO, the big t1 named ought to be at least as difficult and the lower t2 mobs. That would mean the guk lord/king hitting for 800-1000 and would require a group to handle if you were within the level range req'd. VS, for example, should be hitting for 1500-1800 to be in line more with the mobs from solb etc.
In addition, the loot tables for these mobs needs tobe specific and worth the time invested in farming them. I was thinking they could drop pieces from t2 to help with the transition from t1 to t2, or additionally, drop rare augs in general.
I would also suggest locking t0 and t1 to a specific level range. t0 no one over 40, t1 no one over 65 etc.