Boss groups has been adjusted once to count attributes and tiers, so no high tiers can't stay at bosses over powering them. This still needs work it's still possible get high tier on bosses. If you cap your atts half way at n500, it's possible to stay at bosses even higher tiers. One example is at G4, there is one 101 lvl, we believe he has capped his attributes to 3500 or so. T11 is way too high for G4. Attributes makes less change to bosses than what tier ups do. There should be some limit for highest tier possible with n500 attributes.
I will have to analyze the data around this before I can give a commitment one way or the other. That said, I will look into it and run some numbers around grouping calculations with this scenario in mind.
This suggestion doesn't solve the problem. I experienced the same problem in G4, the group is far too wide and we're getting to the stage now, where often boss groups can have quite a few participants in them. There's another problem with g5. I think the defence of the g5 boss was increased to deal with the Strykes problem before g6 was created. I think that both problems can be somewhat solved by inserting another boss group between g4 and g5. I'd need to test this a little to work out how well this would work. IIRC, G4 captchas stats from 3500 up to 4800 (with T11) and G3 sit about right. The problem with G5 could be seen with people like Valty not participating at one point because it just isn't worth the time. Hence, it might actually be good to run a trial for a week or two to see if this solves the problem.