Actually, using the combat tables from the 5FW board games indicates a significant advantage to TL15 vs TL14.

Also, decades ago on this list, ???(can't remember his 1st name) Higginbotham posted that according to his analysis each TL conferred a major advantage that was really, really big. I think it was the MT era at that time.

TUhistory-wise, canon confims that the 3I has been crappy at war for centuries!