When you reference rig
tension, I assume you include backstay use...Assuming truly matched boats, I would think two sources of such a significant difference..the first is sails, if one boat has a newer genoa and/or main, sail performance could make a difference. The more likely difference is sail trim and helming. If you have agood relationship witht he other boat, try try to switch sail trimmers for 10 minutes before or after a race. If that doesn't explain the difference, switch skippers too...and you should get to the bottom of the delta
For a number of years we raced a C&C 30 in a PHRF fleet that included another C&C 30 that had a replacement tall rig
and newer sails. Boat for boat, we always went just a little faster upwind, and there was nothing they could do about it. althought they often placed well in the fleet I don''t think they beat us more than once in 3-4 years.