Even dropping it down one hole may be enough - I (finally) got the Merc out for a test run on Sat, it's also manual trim, so I put the pin in the middle hole , simply becuase it gave a visually similar angle to the old Yam.
Opened up got on the planre and promptly ventilated in a straight line! Back to the marina, pulled the pin, dropped it by one hole, and it now turns sharply with no issues, and bounced over a ferry wake quite happily. Remmeber loosing straight line speed is one thing, but if it ventilates (and therefore slows the boat) at the slightest hint of a wave, loosing half a knot won't affect your average speed.
I would get some lightweight string, tie your trim pin on (so if you drop it you aren't totally b*gg*red), and move the pin down towards the transom hole by hole until it stops ventilating.
If it's only letting go in turns just drop the trim pin in one hole. That will probably cure it. If not keep going 'till it's at the bottom, only then worry about unbolting & moving it - remember if it's still ventilating in the bottom trim pin, you'll want to go down relative to the hull, and that will involve the use of a saw!