For every image now, I used a skydome (just like I do in Poser...lol). I never mess with IBL if I can help it - I've always hated them.

What I would suggest is:

Use Linear mode always if you can. Go ahead and push that 'Estimate Settings' button, but not until most of the image has filled in with colored dots and there's little if any black background showing anymore. It might not be exactly what you want, but it will get you most of the way there ;). Don't be afraid to turn the gain up on your sunlight. With outdoor images with a skydome, mine are usually over 5.0 for the sunlight. Sometimes as much as 20.0.

Laurie