I will join the chorus and recommend the SR Excel if you want a lighter boat - and it's an awesome boat in all conditions. Also, since you mentioned the SEI having a wider seat you no doubt tried out the 1G SEI; the 2nd G SEI has just arrived, and it will probably be a little less stable than the 1G (but faster). The distributed weight difference between the layups pretty much means you can cross from a lighter ski at one level to the heavier layup ski at the next level without too much change, but if you stay in the same layup you'll experience a greater difference in stability. I had a guy test out my 1G SEI in Excel and my 1st G SEL in Advantage last year, and he found he could handle both ok on flat water, so he opted to order a 2nd G SEL in Excel (the 2nd G boat was brand new to the market). Much to his chagrin, he found the SEL Excel so much more challenging he sold the boat a short time later and eventually bought my SEI.
As a side note, it's more the weight (or more correctly, the distributed mass) than the flex, although some flex can absorb a bit of wave action and aid stability. If you try one of Stellar's Multi-Sport layups, you'll find that the stability is very similar to the Excel layups, in spite of being 9 or 10 lbs. heavier and more flexible. That's because the hull is a single layer of carbon/Kevlar weave, making the outer structure of the boat fairly light, but there's a heavier keel to maintain stiffness and absorb impact, so the bulk of the mass is located close to the longitudinal roll axis of the boat. This results in a lower rotational inertia, and a more reactive hull (like the proverbial ice skater spinning and bringing her arms in close to her side; her mass is unchanged, but it's been redistributed closer to her axis of rotation).