I've been to the LAX National several times in the past month and found the selection quite good. I always look for a "sporty" car as well and here's what I found the last time I was there (in the order starting with the best):

Pontiac G8 GT - sportiest car I've rented with National
Impala SS - 300hp V8, leather, XM, etc
Buick LaCrosse Super - same as Impala (haven't rented one of these but will soon)
Buick Lucerne CXS - Very comfy, nice ride, not as quick as the others but a little sporty

FYI - I do not consider a CTS a sporty car. I was so disappointed the last time I got one that I returned and got something else.

Good luck!