Some years ago, there was a publicly available web site called UC StatFinder. Among the things that it showed were average GPAs of transfer students at UCs, relative to their prior college GPA.
From what I remember, the top range of prior college GPA (3.8-4.0) transfers to UCs tended to average 3.4-3.7 at UCs, with some variation by campus. At lower ranges, the difference got smaller, so that those with prior college GPAs around 3.0 tended to average around 3.0 at UCs (obviously mostly at the least selective campuses, not UCB or UCLA where they would not be admitted).
Obviously, individual students will vary in how well they do relative to their prior college GPAs (which are probably in the 3.8-4.0 range for most transfers admitted to UCB these days). But if the trends from when UC StatFinder was up still hold today, it is not likely to be typical for a transfer student at UCB to fall to a 2.5 or so GPA (“Bs and Cs”).