large main or large headsail?
Actually, James, most of the modern fast boats have much larger mainsails than jibs. If you look at boats as diverse as IMS typeform boats, to Open class race boats, to America''s Cup boats, to racing multihulls, to Volvo 60''s, all have large mainsails in proportion to their jibs. Most older rating rule based race boats (CCA & IOR for example), were designed when the rule under penalized headsails and therefore have large jibs as compared to the mainsail strictly as a way of beating the rule. The only modern rule that still under penalizes headsails and so have predominantly small mainsails and large jibs is the MORC rule.
In a general sense, for an equal sail area, a fractionally rigged boat will be easier to handle and will be faster. It is only when you get into unrated area that a Masthead rig shines. If you were designing a rig for speed but independent of rating rules it would probably have a 7/8 fractional rig with minimal overlapping jibs and a sellection of masthead spinnaker and fractionally spinnakers.