I've measured the springs from a Model 3 RWD and the rears were 475 lbs/in.
I ran 800 lb rear springs on a race car for a while.
I'm not sure what 1000 lb springs would feel like, but I don't think the word "stiff" would cover it.
Assuming it was not another Tesla M3, the measured rate has very little to do what the wheel rate is, which is what you care about... Well, you would care more about things like ride frequency, but thats a bit beyond this discussion
For example, if you had your 1000lb spring on a car with a .8 motion ratio(spring travels .8" for every 1" of travel), that would equal a 640lbs(you must square .8 to get your calculated motion ratio) wheel rate, or spring rate at the wheel.
If your motion ratio was .6, your effective spring/wheel rate, would be 360lbs using that same 1000lb spring. Small changes in suspension geometry and design make a big difference with things like spring rates and sway bar rates as well. The actual spring rate of the springs itself means virtually nothing to how it will work on the car.
Suspension geometry and handling dynamics are very complex on cars like Teslas. They are tuned with the on board stability control, ABS etc. Trust only the very best companies who understand these things.
While I dont own any of their parts or know them personally, Sasha at Mountain Pass Performance seems like he has a good grasp of this stuff. I was exposed to him through a youtube channel not related to Teslas before owning a Tesla. The dude is smart.
My background is in amateur road racing and autocross where I went down the rabbit hole on suspension design and theory to make our cars faster on the track.
David