I'm just thinking of the pros /cons for an overstuffed Summerlight, vs an ultralight. It seems that by the time you have a Summerlight overfilled, the weight would be within the ballpark of an ultralight. Ballpark meaning within 6-8 ounces. By using a Summerlight down into the 20's I would need additional clothing as mentioned. Insulated pants, and puffy jacket. I would already have a jacket for those conditions obviously, but could use a lighter one.
I'm thinking that
1) an overfilled Summerlight with additional sleep clothes would be heavier than an ultralight in cool weather (20's)
2)an ultralight in warmer temps (40's) would mean carrying extra fill weight when it may not be needed.
I would say the majority of time, this would be a compromise between the scenarios. For th most part, it seems you can't go wrong with either choice.