Capitol Weekly has done a California Senate Exit Poll of Absentee Votes Cast by emailing people who have voted. (Many people give an email address when they register.) There's no indication that the poll is representative of everyone who has voted but when you have 15,407 respondents you're going to be fairly representative. While the VBM voter tends to be older than election day voters I'm not sure that favors anyone.
Kamala Harris is killing it with 53% of the vote. That's surprisingly good. Most polling suggests she'll finish somewhere in the high 30's. Loretta Sanchez is in 2nd with 14%, leading Def Sundheim with 8%. If this is the true VBM vote, I don't see Sundheim picking up enough points to finish second.
There is reason to question the results. Democrats are leading Republicans 69%-26% in the poll. In previous statewide top two primaries Republicans have only had less than 40% in one race, the 2014 Secretary of State race where a long time Republican ran as an NPP. Even if his votes aren't counted as Republican, Republican candidates got 36% in that one. VBM this year is 49%D/34% R. It was 44%D/37% R in 2014. So even if there is a drop off in the Republican vote I wouldn't expect it to be below the VBM return percentage. If the vote were actually 59% for the Democrats and 36% for the Republicans it's possible Sundheim finishes top two.