Right now it looks like the current top speed of Brightline is the same as most Amtrak lines in California (79mph). Brightline claims in the future it will have speeds up to 125mph. Conventional Amtrak trains already can go 110mph on sections of high quality track, and do in some places in the USA.
True, but the only place Amtrak gets close to that I know of in California is through Camp Pendleton, and there it maxes out at 90.
A "bright line" style upgrade to the SAN-LAX route would have been entirely worth it, and they're struggling along with it, but at pennies compared to what has been spent on HSR so far.