...compared to the popular 255 wide front. I ordered some Nitto 555's at Discount Tire. They are also gonna test fit a 275 wide in the front and 295 wide in the rear to let me decide if I like it. I'm coming off a 255 front and 275 rear. While I know 295's are not a problem in the rear, I haven't seen any comment on 275's in the front. Will it be significantly stiffer to steer because of the total 40mm added rubber contact?
Nothing I ever noticed, and that was with 285/35's on 18x11 wheels. Probably more negative camber than you're running, though.
Some people on another Mustang forum have experienced tramlining with 275/40's (9.5" GT500 wheels, which shouldn't make too much difference).
Is there even a performance advantage or disadvantage to it?
Certainly there is during any hard driving, because the understeer should be a little milder. Over any extended session of hard driving, larger front tires will tolerate it longer before overheating and turning "greasy". If you've got a wide enough wheel, you'll have slightly 'crisper' turn-in response (the car will respond a little quicker to your steering inputs, kind of like it actually wants to turn).
This is going to be hugely dependent on your tire choice, but it doesn't have to be any more of a problem than with the OE size. In the first video, speeds are ranging from about 40 through the slow turn just past the bridge to a little over 110 mph down the main straight on a clear lap toward the end (obviously this wasn't tried on any public street or highway, but it's nice to think how much better off you'd be at, say, 65 mph). Michelin Pilot Super Sports. I doubt that any of the Nittos are quite that good in the wet. For sure, the 235/50-18 BFG KDWS tire that were the OE tires on my car were never even close to that good, not even new. And that's with them being about 2" narrower than the Michelins.
In the second video, you can see that as good as the Michelins are when everything is wet, you can't run nearly as hard as you can in the dry when you hit a wet spot (duh!). Especially with cool temperatures (50-ish) and max-performance summer tires (still the Michelins). Watch very closely for the little twitch around 1:57. No stability control, TC off. The BFG's would have snapped around a bit further except that I wouldn't have trusted them enough to be going that hard in the first place. Tires do make the difference here.
Does the added rotational mass make it significantly worse to drive?
I doubt you'd ever notice it. Dragstrip and autocross timing (to 0.001 second or better) can pick it up, but a difference of 0.1 - 0.2 seconds out of every 40 at autocross or 0.050 second in the quarter mile for 20 lbs more total tire weight is not something many people can feel.
Norm
https://www.youtube.com/watch?v=g6brVFxPSPw
https://www.youtube.com/watch?v=3WGfLOeJvcA
