Has anyone moved from the stock 225/40-18(F) and 265/35-19(R) to 275s or 285s in the rear? I don't want to change to 235s in the front because I don't want more sidewall up there. If going 275 or 285 in the rear causes more understeer, I wouldn't want that either. Overall goal would be for more grip all around, but with a similar balanced handling. I'm guessing my best course of action to achieve that goal would be summer only tires, but I don't have space for an extra set of wheels. Hence, the all seasons. I do drive in the snow, but they ploy here (Chicagoland), so it's never more than 6". Most of the time is less than 1." I also track occasionally. The AS/3s have done all that for me so far.

My current setup:
1999 C2 Cab 996.1
Front wheels 8x18" (Option code 413, turbo look solid spoke)
Rear wheel:10x18" (Option code 413, turbo look solid spoke)

Current Tires:
Michelin Pilot Sport All Season AS/3
Front: 225/40ZR18
Rear: 265/35ZR18

My alignment setup maximizes tire wear (I've gotten 25,000+ miles from my current tires)

 

Increasing the rear tread width is going to add more understeer. I also have 225/40/18 and 265/35/18 and I shudder at the thought of additional understeer.

Going to 275 or 285 will also add to your rear sidewall height, so the rear end will not be as responsive as your current setup.

Your best choice would be to choose a better tire, but given your current tire and your requirements, it basically means upgrading to the Michelin All-season 4.

 

Makes sense. I do like the way my car handles. Currently. I have zero issues with oversteer, and any understeer can be dialed out by trail braking. That, and I’m rarely on the track anyway.

 

285 would mean going to 30 aspect ratio in order to keep factory dimensions. As such sidewall height won't go up but down and together with the wider tyre it will increase responsiveness/lateral side force build-up of the rear axle.
Furthermore, keeping 35 A/R and going to 285 tread width is not advisable in conjuction with 225/45R18 as
1. It will activate MSR within the TC/PSM, which will feel like you've lost engine braking.
2. Will rub at the top of the rear wheel well at full compression. Not sure if relevant where OP drives but still.

 

235/40 295/30 is perfect I have a better ride my braking also got slightly better grip
wider track gave me better high speed hold in the turns and faster exit speed

 

I couldn’t find a 285/30/18 all-season tire on tire rack.

 

That is valuable info to OP's question!
In that case, OP, I recommend to stick with 265 at the rear.