Re: Mods for V1 CTS?
The best you can practically do usually is minor upgrades to exhaust (catback system from Borla or Magnaflow...etc), intake(Volant is the most used on this forum, K&N is an option as well).
Eibach and H&R offer lowering springs.
As AAIIIC mentioned you can upgrade suspension to coilovers but that's definitely expensive like for any car, most CTS owners who daily drivers don't end up with the coilovers on this CTS. V sway bars front and back I highly recommend.
Brakes can be upgraded depending on your wheel size. If you have 18" wheels you can upgrade to V1 brake calipers front and back.
R1 concepts makes very good quality rotors in stock, slotted, drilled and slotted & drilled type. E-LINE or Premier Series
EBC brakes makes brake pads for the first gen CTS and they are probably one the best pads out there.
For street use their Red Stuff pads are good.
I had E-line rotors lasted almost 3 years of hard driving, now I have their premiere Series with EBC Red Stuff, they are sitting in my garage I'll try to get to that this weekend.
But that's mostly it, the stock brakes are decent to begin with, I recommend brake upgrades for those who know will be driving very aggressively on backroads and what not.
The Vsway bars come in very handy with high speed maneouvers.
Also installing the factory V6 strut tower will reduce flex significantly in the car, that I have done as well.