Just thought I'd tell my story in case it helps anybody out.

I live in california. There was a great looking 818 for sale in colorado (already registered and legal road car in colorado), I flew out, test drove it, decided I wanted it, and had it shipped to me here.

First step was to go to the DMV, I told them I wanted an SB100 vehicle registration, and they all looked at me like I was an idiot. Basically, nobody at the DMV knows anything about SB100 by that name. I found a super helpful employee that was willing to look up what was needed. She got me my SPCNS number, gave me the info for CHP VIN inspection and BAR SMOG inspection. I got my 60 day registration (really they gave me something closer to 80 days) and went on my way. Everything I've read is that you do not fudge your claim for how much you paid for the car here. What california really cares about is getting their money, and ethics/laws would imply that you should declare the actual amount paid. So I did that and paid california ~10% of my vehicle price in taxes/fees.
I was under the impression that once I got this, I was pretty much in the clear for my registration to go smoothly, the BAR official changed that impression.

I called the CHP to schedule the VIN inspection, it took a while to get scheduled, I want to say 3-4 weeks, so do this right away. I feared that the CHP would want to do the full vehicle inspection, but they did not. All the officer really cared to look at was the VIN put on there by the colorado dmv. For the record, only reason I feared a full inspection was that I don't have windshield wipers.

After that, I called to schedule the BAR inspection. Once this was ready, I went in to one of my local community college colocated BAR sites.
This was the visit where you give all your paperwork. The seller of my car had excellent documentation, which was a requirement of me buying the car. I had way more than the guy at the BAR wanted. He wanted receipts for frame (factory five receipts), engine, transmission, and maybe a one or two other things. way less than I expected, but he was impressed with my level of documentation. He also took some pictures of the car and engine.
Before I left, he made a point of telling me how bad of an idea it is to buy an out of state car for something like this. Even though I had all my documentation, he acted like it was a coin flip by sacramento as to whether or not I would get my approval. This scared me, i thought everything was great in my documentation and thought it would have been black and white based on things I read previously, but this official told me otherwise.

A few weeks later (I cant remember the exact timeline) I got a call back saying I had my (conditional) approval from sacramento, and now I needed to come back to the BAR for the final inspection. Turns out they wanted me to fill out a few more details in the statement of construction I filled out, but otherwise everything was approved. I went back in, thinking this was when they would do an actual smog test, just for their records as i'd read things saying they would do this. Turns out, they just use the smog equipment to submit the paperwork to sacramento, and then he put the sticker on the frame. At no point did anybody ask me to start the car, but they do expect you to bring the car to them and I feel like someone asked me if it was running. He said I was done, and told me I could finish my registration at AAA if i didn't want to wait in line. I tried AAA (i'm a member), and it turns out he was wrong, they cant do anything related to kit cars.

I went back to the DMV, and they gave me my plates and registration information. It took a few back and forth exchanges between the person I was talking to and sacramento to get the info put into the computer correctly, but in the end there were no problems here.

Received my pink slip soon after. My car is registered as a year 0000, make: SPCNS

All in all, the process took longer than the standard 60 days they are supposed to give you on temp registration. I immediately booked everything, but things are sequential. They wont let you book BAR until you've completed CHP. Wasn't a problem for me as the initial DMV person gave me longer than 60 days.

TLDR/Conclusion:
You can buy a used car that was previously registered in another state and import it into california. This process lets you skip the detailed CHP inspection where they run through the big checklist. If you believe my BAR official, this didn't guarantee me success, and some level of luck is involved, but hopefully he was wrong on this point.

If anybody in/near the bay area wants to check out an 818 before they buy, feel free to send me a message.