The fiberglass spring have more bounce, so the shocks have a bit more damping to compensate.
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I put a 330# composite spring from VB&P on the car a couple years ago with the original shocks, and felt sort of like you do. Researching the shocks situation lead me to use the KYB's due to the damping rate of the spring. Sure, I looked at Bilsteins too, but for the money I couldn't "see" that there would be much difference, and since I've never owned a car with Bilsteins on it I can't make a comparison. But, I can tell you I am happy with the KYB's, and I think you would be also.
It all comes down to how deep your wallet is, or what other do-dad you can live without for a while in order to purchase the more expensive item this time around.
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I've heard bad things about the composite springs. I've heard they don't settle right and the car looks jacked up in the back. I've also heard they get brittle in the cold ? Oh well, I'm not too worried about weight any way, if I was I'd lose weight myself. 
I ordered the Bilstein shocks, look forward to getting them.
Jim
Lifetime Member #73
The Money Pit.... and my niece 
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I was looking into buying the KYB-GR2 from Vette Brake Products(VBP) the price for all 4 were $160.00. For the heck of it I was on the TireRack.com web sight. They where selling KYB-GR2 front and rear for $19.99 a piece. Now I know that there is a different part# for the rear if your using the fiberglass spring. But what is the differance between the GR2 from TireRack vs. all the other places that are selling them for twice as much. Also what is the difference in the valving of the GR2 made for steel vs. fiberglass rear springs.
Thanks Tom
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As I mentioned in the above post, the KYB's from corvette vendors are valved differently than the ones that the corvette warehouses sell. They are valved to react to raod conditions a lot faster. The ones that Tirerack & Summit sell are pretty much just a universal shock that can fit many other applications. A lot of R&R was done with KYB to get these valved correctly.
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Installed the new Bilsteins yesterday...I'm impressed. I'm going to ride down some bumpy. windy country roads today on the way to get the tires balanced. But so far, it's a marked difference and an impressive change.
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The Money Pit.... and my niece
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Vette Registry Founder
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Here's a clickable link to what Van Steel mentioned above.
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BTW,
The ride is awesome and my butt doesn't feel like it's riding the pavement anymore. I think it was worth the extra $$$ to buy the good ones.
Jim
Lifetime Member #73
The Money Pit.... and my niece
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