The tires aren't new, unfortunately, if they have the normal 8-10 miles on them. I've never heard of a "new" tire shop buying back used tires. Try eBay.
As for spring rates, I've found that BMW are sensitive to wheel/tire weight in their suspension tuning. For example, my E46 rides more nicely with the heavy OEM wheel/tire combo than with my snow tires, even though the shows are higher profile. Nonetheless, I don't think you'd have great success trying to re-calibrate the suspension yourself with aftermarket parts, and it certainly wouldn't be worth the money.
I think you'll find a huge improvement from dumping the RFT's without bothering with anything else.