Michelin PSS AS3 vs. AS3+?

[JHB(Audi?)]

Can someone explain the difference between the normal AS3's and the AS3+'s to me for my 2013 245/40/r18 A4 quattro?

I'm about to head out on a 2000 mile road trip, and I've never been terribly fond of the stock P7's. I've put about 15000 miles on the thing in the fifteen months since two of the tires were replaced (at CPO purchase) and so I figure this might not be the very worst time to stick a fresh set of rubber on the thing, especially since the other two tires, regardless of wear, might be getting a bit long in the tooth.

I still have plenty of tread left on all tires, per the penny test.

Should I spend the money to replace the tires now? If I do, which do I want? Is there any difference in a car that will most likely be spending much of its time on 85MPH highways?

I'd like to save money if possible, but given that I'm going to be driving through the depths of Arkansas, for example, and with a fair bit of extra weight packed in the car, I'm starting to think it might be in my best interest to get fresh rubber. Thoughts?

[A U D I]

AS3+ is a little better in snow weather. I would find out how much your tread depth is left and go from there. Sometime looks more or less so I always use a tread gauge to know for sure. You might be fine with what you have now.

[JHB(Audi?)]

P6's, not P7's.

I'll go pick up a tread gauge today. Really, I'm looking for an excuse to get rid of these chattery, noisy, bumpy things before the trip. I realize that's stupid since the tires will likely get MUCH hotter in the course of several 8 hour days of driving than they normally would, and will be heavier than they normally would, so my new expensive tires would be wearing extra quickly, so if I'm going to replace them I should replace them after arriving rather than before.

How would you determine whether or not you'd "be alright"? Clearly, you want a higher threshold than the standard "4/32 = safe" with long periods of rural driving and a donut spare, right?

[A U D I]

That's a good reason for the ride comfort for sure. 2k miles in a car ride is a bit if you have tires you don't like and have the money to replace them. I wouldn't think on the temps too much because, yes hotter temps will make most tires wear faster but unless you are desert driving or on really rough asphalt it shouldn't matter so much. Tires also wear less when they are new compared to lesser treads in most cases. Just make sure your tire pressure is good. I set mine higher to help tire wear and MPG (40psi depending on tire and car).
As far as your current tread goes, I think you are good at over 60% if you might be driving in rain any. New tires are always safer but lower speeds can keep tires under control if needed.