Obviously, you aren't supposed to be going faster than the posted speed limit, however the 15MPH over "rule" is not law (i.e. not specifically stated in the CA Vehicle Code) - no matter what the CHP said. I know CHPs myself and I've heard that before from many of them - it's basically a "rule of thumb" that they use to differentiate safe vs. unsafe overtaking of a vehicle, which is what they would most likely cite you for, if you were going too fast past vehicles while lane-splitting (assuming you weren't going faster than the posted limit).
When lane-splitting in SoCal - this seems to be the CVC rule (as most lane-splitting is done on freeways between the #1 & #2 lanes):
21755. The driver of a motor vehicle may overtake and pass another
vehicle upon the right only under conditions permitting such movement
in safety. In no event shall such movement be made by driving off
the paved or main-traveled portion of the roadway.
It's all pretty relative though - if you _are_ going faster than 15 MPH over traffic flow, chances are you're probably splitting lanes too fast (i.e. unsafely, if there is a "safe" lane-splitting model).