Maybe not in the sense of "phew, wasn't sure if I was going to make it", but definitely a challenge as a TT. Many East Bay riders know their time on the bears, Grizzly Peak cyclists do a monthly time trial on the loop and several races make it a central feature including today's Berkeley Hills road race.

OP - IMO you should just go climb Diablo - with the exception of the last short stretch to the summit, most of Diablo is no steeper than mama or papa bear.