OK, went back 3 years and looked at as many Sophomore performances as I could and compared vs their Sophomore year in the NBA. There are lots of examples of Sophomores playing well in Summer League and not doing very much their Sophomore year in the NBA. I couldn't find very many examples of Sophomores who played poorly in summer league having a good Sophomore season in the NBA, but I would also caution that many of the top players don't play summer league, so there isn't a lot of data there.
I would still probably conclude that Summer League doesn't mean very much, even for Sophomores, but if anything to not get excited about a good summer league from a Sophomore and to be extremely skeptical of a Sophomore who plays poorly.