I have replied to a mail from John, but to keep the discussion going – here are the main points of that reply:

3" seem a little too much. You may end up with the lateral center of gravity forward of the lateral center of boyancy, which must be avoided in a rudderless kayak. Such forward tilt may increase flatwater speed slightly (and is thus favored in racing kayaks and, to some extent surfskis) but will play havoc with directional control (over-steering in automotive terms). With the LCG aft of the LCB the kayak becomes easily controlled with predictable movements (slight understeering). Small errors in the LCG/LCB configuration can be quite easily compensated by moving a water bag and other heavy items forward or aft until the kayak behaves the way it is supposed to, but too much may make paddling unnecessarily challenging.

Ideally you should move the cockpit rim aft 3" instead of the seat forward 3" – but that is of course quite another undertaking.

Consider when you have the seat out, to laminate the aft edge all the way up to the rim (as suggested on the plans). It is incredibly comfy for layback rolls and braces and it supports the lower back without hampering the upper body rotation.
You may also find that if you can slide sideways an inch or two in the seat, many rolls and braces will be significantly easier to master (imagine the kayak on edge – if your hips can slide down an inch towards the water you find a stable floating position quite easy, while if the sides of the seat forces your hip to remain centered, your weight will poush the kayak on top of you and things will be more complicated)!