My 81 Bianchi Superleggera was built with SP downtube and chainstays and SL in all the other tubes. In my opinion, I've never ridden a better bike. My 78 Tim Isaac was all SP and it was my second fave.
I weighed between 165-175 while riding these bikes and always considered myself a "heavier" rider. The off topic DeRosa Merak, currently on the market and a superlight alu racing bike posts a rider weight limit of 165lbs, so that may be a point of reference as to what is heavier and lighter with regard to rider weight.
Donald F. Dunstan
Winters, TX