If I had 700/800 tracksport springs, would you trade your 600/700s or similar?

They're pretty cheap. I love my 675/750's . . . =)

Where to buy? APR?

You have motorsports, though...

and . . . 10 inches up front and 8 in the rear, I think there's a t-shirt like that . . . APR,
 

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STaSIS, Hyperco, Eibach, Brits . . .

And I have the 82XX series Koni Motorsports which are dimensionally similar.

are you really going to notice a 100lb difference?

A couple people have mentioned problems w/ underdamping at that high of a rate
 

BoneStockS4 &amp; another, IIRC

I'd say so. I noticed a significant difference when I went down 150lbs.

Yeah, I had to increase the rebound way up before I was happy with straight line driving...
 

Unfortunately that made it very harsh. Dropping down to 550/650 springs made life much better.

Swapping over to Ohlins with my old 700/800 springs was probably the best mod I've done :)

No...