I don''t know how much in vogue the "slot effect" theory is these days with racers or theorists. Sailormon is probably correct in saying that making the "slot" too small will reduce lift of the headsail/mainsail combo. The interaction of headsail and main is one that has certainly generated a lot of "wind", but Sailormon is certainly correct in generally arguing that you have to consider the two sails as working together, and try to optimize the performance of both for best performance of the boat.
You don''t say whether your boat has a boom vang or not (a control line which allows downward force to be exerted on the boom, usually attached between a point near mid boom and a point near the base of the mast). The presence or absence of a vang will determine how much use you can get out of your traveler.
Without a vang, the traveler is your best way to control the angle of attack between the main and the wind, particularly when beating to windward. If the wind shifts and you compensate by easing the sheet, your sail shape will change. If it was well shaped before, it will no longer be so. A vang-less boat does best to play the wind shifts (or adjust course) using the traveler to keep the mainail''s angle of attack optimized.
A boat with a vang on it can "vang-sheet", usually in a stiff breeze: tighten the vang to flatten the sail, and use the sheet to control the sail''s angle of attack. If the vang is set rather loose for light winds, however, your sail shape may change if you sheet in too tight in an attempt to get the sail angle (boom angle) close to the centerline of the boat. In light air, a traveler is useful to allow a full shape in the main (having an eased sheet) while pulling the boom to windward until it is centered for good pointing ability.
To some extent downwind, the traveler will also replace the function of a vang, allowing you to shape the sail with the mainsheet while having the proper angle of attach relative to the wind. Alas, most travelers don''t have enough range of motion to allow a fairly broad reach with flattened sails; this is where having a vang is a real advantage.