Have the stock non-powered seats in my 04. While they provide great lateral support after 45 minutes or so without lumbar support my back would start to get sore. If I tilted the back far enough to provide lower back support then my shoulders had no support as well as it put me in poor driving position.
It was to the point I began looking to replace the seats. $3k Recaro seats seemed to be the best solution when my better half suggested I try some type of pillow or cushion first.
So a couple cushions later I have found one that works fantastic for me. It is a manual air bladder type that is 6"x11". Fits great in the seat and straps in the back to hold in place.
I have no affiliation with this other than it has made it a joy to drive my car and may help if someone else has similar issues with the stock seats. BTW it cost $25 so my wallet is happy too.
Lumbar Air Plus Pad
http://www.businesscopiers.com/backstore/air-back-support.htm
If someone can figure how to get the seat covers off I would be interested. Would put this under the skins if I could figure out how to do this.