I would put them about even. Equal cleaning area (system, except for rethresher, in fact). The small rotor CRs seem to be limited by the rotor diameter. Had a 1st year 7010 demo against our TR98 and they were about even in every way (except cab comfort!). They had a HP update after that demo, but it was dry conditions on durum, so hp wasn't a huge issue.
It all depends if HP is limiting or sieve area. If you are running in a heavy, dry canola crop, the cleaning area is most likely limiting capacity so the big machine would be better. But in cereals or peas, probably the machines would be very evenly matched.
Why do you think the chipped 7120 will out perform the 9070? Its still a 9L eng vs a 10.3L. Totally different torque curves, power rise, etc. I think you will find that:
1) You will run out of sieve capacity.
2) A chip may add 35-40hp but as the old saying goes "no replacment for displacment"
Sounds like a good place for a 9065, that would be the equivalent of a chipped 7120, or a close as you will get.