Let me check my math now ... Anaheim to LA Union by Amtrak timetable is 31 miles. Assume at least one stop in between for all services (wasn't one of the complaints about a HSR station location in Buena Park?).

Current Amtrak timetable (stops only at Fullerton): 40-45 min. = avg speed (worst case) 41.3 mph
(Max. speed between stations ~79 mph)

Assume 110 mph max speed: with 1 stop at Fullerton, average speed probably wouldn't be more than 55-60 mph. Cuts Anaheim/LA travel time to about 31-34 minutes. Could be done with existing tracks using better signaling, a few modest realignments and probably a dozen grade separations, and refurbished existing equipment. If you want to throw a wad at it, you could probably do the whole thing including an extra track or 2, wires, and suitable clearances/platforms for HSR equipment or severely upgraded surfliner gear for $1B or so.

Assume 200 mph max speed: can't be done with rail acceleration characteristics in the miles available with one stop - max speed would probably be in the 125-150 mph range and even that would only be for short periods. If non-stop, would increase avg speed to 100 mph or so - let's assume 125 mph for grins. Cuts Anaheim-LA travel time to 15 minutes, but if you tried to do that with a stop in Fullerton or Buena Park-Norwalk the passengers would need seatbelts or roller coaster chairs with locking bars! More practical figure might be 20-25 minutes. So compared to 110 mph max for $1B or so you're saving maybe 10 minutes more for probably another $2B.

I like saving the 200 mph stuff for open country and doing LA-Anaheim on upgraded Surfliner tracks. If HSR isn't eventually going to San Diego along the coast (and by all indications it's not) it makes no sense to invest in full-scale HSR construction for 30 miles.

Practical running time even for expresses from BA (say, San Jose) to LA Union, considering speed restrictions in urban areas, some curves, and tunnels/grades would probably be on the order of 3 1/2 hours - maybe as little as 3 with an express or 1-stop limited and the "base tunnel" route (which isn't really a base tunnel: it's a tunnel on a 4%+ grade up to Gorman, a brief daylight section crossing the S.A. Fault, then another tunnel on similar grade down to Santa Clarita - probably at least $5B more to build than the Tehachapi route - is it worth $5B+ to save 1/2 hour?). Either way it's competitive with air travel considering current terminal times (especially that 2-hour security hit before departure). The 2 1/2 hour times are a joke - not doable in the real world with real grades and alignments and real-world equipment.