What about a new system:
T1: 100%
T2: 115%
T3: 130%
T4: 145%
Personal bonus
T1: +5%
T2: +10%
T3: +15%
T4: +20%
T5: +25%
So If you have nothing:
100%
If you have max group tier of T4 and max personal of T5
you get 100 + 45% + 25% = 181.25%
if you have Group T1 and personal T5
you get 100 + 25% = 125%
So as long as you try really hard you are always making over 100% making less than 100% or on 100% will feel kind of depressing like you are under-achieving.