There is no maybe about sub-frame bush upgrades. They are a foundational component of the 5th Gen suspension. The IRS shifts under loads causing a jittery rear end also known as rear end step out. Nothing corrects rear end step out but sub-frame inserts or full sub-frame bush replacements. Adding sway only makes rear end step out worse unless the sub-frame bushes have been addressed. The same is true for better wheels and tires, coils or increases in RWHP. Foundation, foundation and foundation. For a daily driver that you don't want to turn into a race car inserts are the best path.
Next on the list has to be the front radius arm bushes. These are hydraulically damped and allow unwanted changes in dynamic castor. As you drive down the road in your bone stock 5th Gen you can actually feel this as you traverse the freeze thaw joints. It is a very subtle, very slight shudder in the steering wheel. We can correct that with full bushes or inserts. For a daily driver that you don't want to turn into a race car inserts are the best path.
Now for the lowering. My guess is that the lowering is as much for looks as it is the handling. Either way, daily driver ride quality is essential. Of course Pedders has lowering coils and they delivers the look and improved handling. Having typed that there is no company that makes a set of lowering coils that ride as well as OE including Pedders. HOWEVER, for less than the $2,000 mentioned in your OP you can have the benefits of a Street Xa package.
Pedders coilovers adjust height by screwing into or out of the lower mount. This preserves what GM call jounce travel. Jounce travel directly relates to ride comfort. With Pedders Xa coilovers you will have the same level of ride quality at close to OE height or dropped on the ground. The damping is adjustable and matched to the coil rates. It is fixed ratio bound and rebound adjustment. Set to two or three clicks off full soft you'll have a ride that some describe as better than OE. At two or three clicks off full hard you'll have a track ready suspension.
Included in the Xa Street kit are the sub-frame and radius bush inserts you need for your 5th Gen foundation. With the alignment bolt kit you'll be able to add castor for quicker return to center of your steering wheel, front camber adjustment is made easy with the included camber screws and you'll be able to adjust the camber on your lowered Camaro to half the front camber reducing understeer and improving rear tire life.
Radius Bush Inserts
Full Alignment Capability
There is no better value for a 5th Gen suspension kit. The only remaining elements are sway bars and differential bushes. Do you really need them with a Street Xa kit installed. My answer for a daily driver would be no. Out of the showroom the 5th Gen ran the 'Ring' faster than some legendary European 'great handling' cars. The ZETA II chassis is an excellent one. In reality you don't need any suspension upgrades, but we all want more. It is the American way. I am so confident that you will be thrilled with your Street Xa kit on your 5th Gen I will guarantee it. If you are not thrilled with it I'll refund your money.
If the Street Xa kit is more than you are willing to spend we start with the Street I sub-frame and radius bush inserts. This covers the foundation of your 5th Gen. A set of Pedders lowering coils would be next. If you have some left over cash we can add a sway bar package. Or we could make it really cost effective as we did with JProberts. Sub-frame inserts and a front bar. JProberts later added Pedders lowering coils.
You'll find the full thread here: http://www.camaro5.com/forums/showthread.php?t=98362
May I also suggest you read the 5th Gen Suspension Book
. You have a lot of options that are all cost effective. Let's makes certain you spend your mod money only on parts that are important to you and your driving style. Let me know if you have any additional questions.