I assume, you rigged your pursuit on the correct MS mast to specs (measured from the top of extension ring to bottom of the furthest pulley), you weigh the same as your mate, used the same board and fin as your mate, and are at about the same level as your mate. If any of the above does not apply, that could explain why you felt at the limit, and your buddy did not.
Did you switch boards with your buddy?
I weigh about 160lbs, and always have to over downhaul my TR race sails by at least 1 cm, for them to feel light.
I also have a buddy (about the same weight as me) that has iSonics, and he is always 0.6 to 1 m bigger than me. I could not handle it (Exocet boards), but I plane earlier than him on the smaller sails.
Numbers are just that: A number to start from.
Good luck !