Sidewall is a must. Im running 255/35/18 rear 225/40/18 front. I have been eyeballing some 245/50/18 all around. The rims are style 71 chromadora rims. Two piece setup, 18/8 front 18/8.5 rear. I have hub rings coming.
I take it that's a BMW fitment? 245s all around will definitely help the performance and the look - those 225s up front definitely look goofy!
You could maybe do stock size 235/50-18s up front and then do 255/45s in the rear, just to accentuate the staggered look. Unfortunately, neither of those combos has a ton of good options in the performance tire realm.
Far as my suspension, im going to upgrade from stock only cause I have FE1 suspension. Just some nice bilsteins, slotted rortors maybe, I want the brakes a little more "grabby".
Unfortunately, I'm fairly certain nobody makes any performance-oriented replacement dampers (like Koni or Bilstein). The only option is to go to coilovers, which obviously can be expensive. You can get cheapy ones from KSport or D2, maybe others. KW makes their Variant3s for the 2nd gen CTS, but they're pretty damn expensive.
If you're interested in the KWs, you might try contacting Belltech. I always thought they were just a sport truck kinda company, but I guess they've branched out some. For the 1st gen CTS/CTS-V, they sell the KW Variant3 setup (35263001) as Belltech 21060, and their price is about 20% lower! Exact same parts, comes in a KW box and everything. Anyway, KW makes the Variant3s for the 2nd gen CTS, but I don't see that listed on Belltech's website; maybe if you call Belltech you'll find that you can get the KWs thru them at a better price.