The thing that limited my weighted chins is that I kept slipping off the bar (past tense). My grip just gave out. I began grip work with 3 sets of 3 on the COC #1 on 22 August, three times a week on a MWF schedule. By the end of October (about 40 exercise days) I was up to 5 sets of 5 and upgraded on 07 November to a 1.5 gripper. It's now been two weeks (at 5 x 5), and I can close it several times with my right hand and once or twice with my left.
The thing that seems to have made the difference is that I now hold the gripper closed (as much as I can) for a total of one minute (each hand) per exercise period. So in 25 compressions (5 x 5) I just randomly pick 6 with each hand and hold at maximum compression for 10 seconds each time. That's the only thing I'm doing different now than with the COC #1, and I think it's helping.