RE: 70 cm fin or after market boom head?
Ummmmm..... better than what?
The gear you have should be as good upwind as anything on the water besides a full or Formula race board (F-160 or 161) or the F-161 based FE board.
I'n not sure how you expect an aftermarket boom head (HPL or Streamlined I imagine) is going to give you significantly more upwind angle or speed than a stock boom head.
Yes it could give you a very slight increase in angle and speed, but unless you sailed both rigs tuned exactly the same, it would be very hard to tell any difference.
Since the aftermarket boom head "feels" more solid, I suspect that you would get a tiny bit more speed and angle from them, but since I hope you aren't trying to beat the formula guys at their own game, for free riding on the Free Formula 158 (2004 model ......right?) you may have reached the limits already.
Rig your sail real "fat" for better bottom end, and then adjust the outhaul to quite tight (giving a very fine entry on the sail) for going upwind.
Maybe move the mast foot around a bit (at least try it all the way forward and back) to find the sweet spot for best upwind angle and speed.
Hope this helps,