The stretchsit works well, but not up north wearing heavy coats or layers while driving. I'm looking at aftermarket driver seats like Recaro or Sparco. Any recommendations?
Unfortunately, aftermarket seats, if they meet American regulations, have the same problems as regular car seats. Most smaller, after-market modifications to car seats include a "lumbar support" that works very hard to put an arch in your back that shouldn't be there. The two brands you mentioned have enormous shoulder bolsters that force the back into a hunched, rounded position, so that's no good, either.
Stretchsitting doesn't work with heavy layers because there's too much stuff in between the cushion and your spine. Unfortunately, a differently shaped seat won't solve that problem. If you really need to wear a lot of clothing while driving, then it's a good time to practice stacksitting, and engage your inner corset. If you're driving for long periods, you might even consider wearing a real corset, to help keep you tractioned. It's always better to use your muscles if you can, but if you have to sit for a really long period, stretchsitting isn't an option, and you can't stacksit while driving, then a corset isn't a terrible solution. It's better to be aligned and relaxed than slumped and misaligned, which puts unhealthy pressure on your disks.
I hope that helps — happy driving!