I took a quick look at the San Joaquin schedule. It's a straight 4 hours between Transbay Terminal and Madera, with about another 2:45 to Bakersfield. Exactly how much would acceleration to 125 mph between Madera and Bakersfield shave off the trip, allowing for diesel acceleration and 4% grades on overpasses? 80 mph is 64% of 125 mph. 2:45 is 165 minutes. 64% of 165 minutes is 118 minutes. This says the best case for San Joaquins rerouted onto HSR infrastructure at 125 mph is a 47 minute saving Transbay to Bakersfeild, but I would bet that diesel acceleration, grades at overpasses, and time at station stops that wouldn't be cut by higher speeds would further cut into any advantage here.