No matter which direction you go, make sure your tires have a load index rating of 99 or higher. One of the big reasons you'll get flat spots is due to the load on those tires that aren't rated for it. Another thing to consider is if you curb your wheels (yeah, yeah, I know, we're all expert drivers here, only our wives/valet/etc have ever rashed the wheels), the softer ply of the tires that have a lower rating will be more likely to gash the sidewall at the seat bead, ruining your tire.
I understand that with a higher load rating, the ride is a bit harsher. If that's a major concern, I don't recommend upsizing your wheels. On 19's, your exact (within 0.25%) match tire options are:
255/35/19
295/30/19
Either of which has roughly the same height of sidewall (~3.5") compared to the roughly 4" of sidewall that you'll want on an 18" rim. That half-inch may not sound like much difference, but in ride quality terms, it makes a noticeable difference.
You're well on your way to really "getting" what is important - I see you talking about contact patch. Yes, the more contact you're making with the road, the better. That said, your stock tires make contact at 10" wide by anywhere from 4-6 inches long patches. Going in the median of that, that means ~50in^2 per tire contact. Going with wider tires adds very little to that patch (this isn't some old Honda, where you're going from 145-wide to 225-wide, net gain 50%) - talking about going from a 255-wide to a 275-wide is a net gain of ~8%, and softer ply means you'll make a bit more contact length to the tune of maybe a percentage or two. Call it a 10% gain in dry, straight-line contact. Having talked with folks who've done big builds on their vehicles, stock tires have done them right up to 550whp. They can floor it and get a chirp as the Quattro kicks in and mangles their Torsen diff. If you're making anywhere near stock power, you're probably not going to need that additional contact patch. Where this all changes is if you're trying to get better cornering, wet, or snow traction.
For cornering traction, brother, you are in the wrong vehicle. 4400lb curb weight and a tendency towards body roll are not your friend. Anyway, you'll do more for yourself swapping in the Hotchkis sway bars than any amount of wider tires could do for you. You could also look into spring limiters for a stronger damping at the extremes of suspension travel. I'm not an expert by any means, but I play one on TV...
For wet and/or snow traction, it's all about the tire itself. Wet traction, you want a traction pattern that does 2 things - maximize the contact patch with the road, and minimize the amount of water allowed under the tire (usually, via ducting out existing water along chevron-shaped grooves that give the water a path of least resistance pointing outward). Snow traction is a whole 'nother beast, and my only recommendation here is Blizzaks for everyday, and chains/studs if you're talking deep snow with possibility of ice. The Blizzaks will wear faster than your typical passenger tire (24k mile rating compared to 40k+), but on an AWD system like we have, it's like a mountain goat compared to rolling around on bowling balls in very slick conditions.
I've gone and gotten all wordy again. Let me know if this helped/hurt/didn't make a difference. If you've got any questions, I'm here more than I probably should be, but it never hurts to PM me and say "Hey, I posted a question in the tire/wheel offset thread, could you take a look?"
Thanks
O_E