I had a 1987 20' Donzi Minx with a 270HP/350 mag alpha... She would run 62-63mph at 4500rpm with stock 23 Mirage.
Changed the cam to the Merc. 280TRS or 300 Tempest (got the specs. from Merc.); no head work required.
Re-jet the carb to the tempest specs. (again from Merc.)
Edelbrock performer Q-jet intake. K&N Filter. Labbed the prop.
With a light load, she would touch 70mph at 5200rpm...
I spent around $800 for a 7-8mph increase.:)

Wish I only had to spend that now a days...