I agree with ocnblu about having a separate name for Buick's performance line-up.
I think it would be good for Buick to keep the CX, CXL, & CXS trims for their normal models, and create a "Red-Line or V-Series" like model line-up that co-exists with the normal Buick line-up. I really do like "Ultra" but I could see GNX also... as in LaCrosse GNX, Lucerne GNX, & Velite GNX. The GNX or Ultra Series would need more than "performance" to set themselves apart from the other divisions. Make the GNX or Ultra series a boutique series and more customizable than Redline or V-Series. (being able to customize is a luxury service expected by a Premium brand)
For the GNX series, Buick could provide:
Interior:
Two tone leather interiors with suede inserts
Remove the standard walnut wood trim and experiment with marble veneer or Corian*)
Add satin nickel finishes inside (I actually prefer Buick to try something different: try experimenting with copper colored trim)
Exterior:
Unique exterior metallic paint colors
Tinted windows (maybe add a unique color to the factory tint that will compliment the exterior/interior colors which will help create a signature look)
Keep the same general shape of the new "Velite inspired" grille but switch the front grilles from vertical bars to chrome horizontal bars similar to Infiniti's
Two or three wheel options
Add bi-Xenon headlamps
Add LED tail lights
Increase HP by SuperCharging or Turbo: 3.6l SC (maybe 290hp-300hp), 4.6l SC 320hp-330hp)
* look at the possibilities with Corian (rumor is Lincoln is considering to use Corian):
OH! And welcome back Paulie!!!! :)