The implications for the Senate are obvious: 10 new Senate seats, four of which would almost certainly go to Democrats. The other six would be competitive, especially in presidential election years. If Democrats and Republicans were to split the six competitive seats, Democrats would net four seats in the Senate.
Cohn has fallen pray to the same thing many national pundits do, judging California elections based on Presidential numbers. The two aren't as correlated as they think. I looked at this in April, with the Feinstein-Emken numbers. Dianne Feinstein was an enormously popular sitting senator with a large war chest running against an unknown Republican with no money in a year where Barack Obama got a record Presidential vote. Feinstein won by the largest margin any Democrat has won by since 1986. So it's a high water mark for Democrats. Feinstein didn't win either Central California or Jefferson and if Feinstein isn't winning either in a good Democratic year, no Democrat is. She narrowly won South California, so that's the only new state they'd have any shot at. If they won that seat in a Presidential year, Democrats would get 7 of the 12 seats, no net gain over what they have now.
As I said, those Feinstein numbers are high water marks. While there are no elections in Six Californias now, there is the Board of Equalization. That divides California equally four ways and in a very helpful way. First, BoE 4 almost exactly mirrors South California. The only difference is that South California has all of San Bernardino county while BoE 4 has about half of San Bernardino county. While we don't have general election results for this district, we do have primary results. The Democratic party didn't bother to field a candidate, although one Democratic "some dude" did run. Republicans won the district 66%-34%. Democrats usually do better in November, but any district the GOP won by 32% isn't going to be competitive. Now it's possible that in a Presidential general election year with an actual well-funded candidate Democrats will do better, but they've generally done poorly in Board of Supervisor races in these counties. Their electoral success has come when the counties are carved up and heavily Hispanic districts have been created. That won't happen in a statewide election. Winning a senate seat would be an uphill battle for any Democrat.
The new state of Central California would be entirely in Board of Equalization district #1. The parts of BoE #1 that wouldn't be in this state are the more Democratic parts, mainly Sacramento County. Jefferson is in both districts #1 and #2. As with BoE #4, the Democratic party isn't contesting BoE #1. They again had a "some dude" who decided on his own to run. He lost 60%-40% in the primary. That's not as bad as BoE #4 but if we only take the part of BoE #1 that's in Central California, we get a 63%-37% Republican win. So a U.S. Senate seat looks like a longshot.
Jefferson is in BoE #1 and #2. Republicans didn't run a serious candidate in BoE #2, just like Democrats didn't field a serious candidate in BoE #1. So it's a fairly equal trade off. The Republican candidates in BoE #1 and #2 won Jefferson 53%-47%. That doesn't seem as bad for Democrats and it's a gap they could easily close in a general election. The problem with that is the candidates. The Democratic candidate in BoE #2 is from San Francisco, an area of strength in that seat. They don't have the same strong candidates in the Jefferson portion of the district, while Republicans have a strong bench in the Republican parts of Jefferson. I wouldn't be optimistic Democrats would win a senate seat here either.
The most likely result for Six Californias would be 6 Democratic and 6 Republican senators, with Democrats having a shot to win an additional seat.