I know this is a late post to this thread, but I'm sure others will come along from time to time as they research the gaff rig vs Marconi debate elsewhere - that's how I got here.
I too have a Tahiti Ketch, but with an all Marconi rig. This was done by the original owner in preparation for a voyage to Australia from California many years ago. Over the years she has kept this rig and I, the 5th owner. have kept for the time being. The original owner was not a romantic in love with the "old world charm" of the gaff rig. I think he simply wanted a rig he could single-hand in mid Pacific.
My needs are much different, just a gentle cruise every now and then, and yes I am a romantic when it comes the old gaff rig. Interestingly though, the original owner did buy another Tahiti some years later with the designed rig of a gaff main, a club footed jib and a Marconi mizzen. When I asked him for an appraisal of the two rigs he confirmed all of what has been said on this forum. The gaff is not as good to windward as the Marconi, but off the wind she has some driving power. The extra halyard and any other mods for the gaff were not an issue.
So far I have found that the Marconi works well and she performs well into the wind, downwind is nothing to write home about.
I have made a decision to remove the Marconi Main and rig her with a gaff on the main. I will also be adding a gaff top s'l for lighter winds, and I will also be rigging a gaff vang. I have heard mostly good comment about the effectiveness of this feature. Only on local person express concerns that the vang could cause mast damage if used in strong winds because of the levering action up high. I guess the moral is to use with caution.
When finally set up as I want she will have a gaff main, Marconi mizzen, and a combination of head sails. Most of the time I'll be setting a stay s'l, a jib and a jib top s'l, but these can be interchanged with two sizes of Genoa.
I certainly would be interested in hearing from fellow Tahiti owners. If any of you are contemplating going the other way - Gaff to all Marconi, get in touch as I have the original sail plan drawing for this set up.