Hey Waverider, congrats on the new car!
When I bought my SR in April I had a weird firmware that no one else had as well, turned out it was because my car was one of the first HW3 cars and that required a different firmware from everyone else. It's likely you have all the latest and greatest features already. If not the next update will probably align you back with the rest of the flock (it did for me). Honestly it's not a big deal to be on the latest firmware for Teslas because half the time the new updates have issues you'd rather let the more enthusiastic fellow owners iron out anyway
Since you have 250mi range I think the reason you have a rare firmware is either:
1. Your car has physical changes from 2019 SR+ models that gets you 10 more miles and as such needs different firmware to run.
2. Your car has a special firmware because it's a 2020 SR+ model and the software improvement is giving you 10 more miles than the 2019 model.(some legal reason to match EPA stated range in car)
Either way I think you probably have the 5% increase if it says you have a power increase in the patch notes.
If you can test the supercharger rate, try to run the car down to 20-30% and plug it into a V2 supercharger (without a neighbor so you don't share power) or a V3 supercharger and see if it charges above 100kW. That would likely mean your 2019.35.xx firmware has the improvements from the 2019.40.xx firmware.