I will be honest and admit to not knowing much in spring rates, etc.
I hate the way the 07's feel, they tend to be bouncy, and at high speeds feel horrible. I want to have the hard suspension the 03-05 have. Can I achieve this just by changing out the 07's springs??
If so then, a nice reply would be accepted with any recommendations.

 

My '03 track responded well to Eibach Prokit springs designed for the 350Z. They lower the Z about an inch and are progressive, not linear, to limit the harshness.

Depending on what you're willing to compromise on (handling vs. ride, for example), everyone has a favorite spring to recommend. The Prokits are well-made, relatively cheap and available...

 

Contact one of our vendors, and ask them to set you up with stiffer springs.

 

+1 on the Pro-Kits! But they are more of a "ride-quality" spring vs. the "hard" or performance type of spring you mentioned you're looking for. For a stiffer suspension, change the struts as well as your springs. Depending on how low you want to drop your car, the NISMO springs are pretty stiff.

If you search for spring rates, there's a good compilation of springs, prices, drop height, and rates on here.

When looking at spring rates, you have to take the drop in consideration. That is, if the spring rate is much stiffer than OEM but drops much lower, then it may not be that harsh as the spring rate (higher) is only compensating for less traver (lower ride height). Some struts are adjustable in ride-height and compression and rebound as well...just depends on how much bread you got to spend on suspension components...

 

bouncy sounds like a dampening problem. Is it more like an oscillation when you're driving? because that signifies underdampening by your shocks. You might want some aftermarkets.

 

I would just go with a full aftermarket setup. I'd recommend Tokico D-Spec. I really think they're the best full suspension setup in terms of bang for the buck out there. 16 ways adjustable gas shocks 375/375 linear springs and dropping 1" all around. I just got them installed 3 days ago and I'm already in love!