i looked into this with the 225 and from what kev hippwell told me,,,it works out the same as 9 x 48 for the 125cc
and 9x42 for the 225cc...
I think for the 125cc it would be worth it as there is less drag and this would give a little more help with stalling the bike at low revs..
It will also pickup quicker from a stand still..
I never went with it on the 225 as there was a load of prower from the motor away..
Good luck with it...