Moving the traveller to the cabintop won''t really do anything the boats performance. It will, however, introduce significant additional load to your boom because the sheet would then be pulling on the center of the boom rather than at the end as with your current configuration. This is why boats with mid-boom sheeting usually have three sets of blocks at different locations on the boom - to try to distribute the increased load better. You''ll also need a mainsheet tackle with a greater purchase than your current end-boom mainsheet, which means a longer mainsheet (or a mainsheet winch). Same thing goes for the traveller itself; the new cabintop traveler will need greater purchase and longer control lines.
How big is your boat? This could be a considerable engineering problem. You will need to be sure that the boom can handle mid-boom sheeting, and that the deck structure can accomodate the traveler loads. Either or both will likely need to be extensively modified.
Personally, I dislike cabintop travelers and would rather have it on the transom (as with my current boat) or bridgedeck. I like having the mainsheet accessible from the helm.