I doubt you'll find anyone with regrets who's installed it on a CTS. With an engine that freely climbs to the redline, it makes a big difference (you can feel). Driving a truck that has peak HP around 3400 RPM, you will have a tougher time feeling it. The hardest part is convincing one-self to cough up $250 for a cold air intake.
The K&N open box filter, has notorious sound issues. Some like it loud, but most will deem it un-Cadillac. The Volant makes about the same power, but is much quieter, since it's essentially the same filter but with a closed airbox and cold-air snorkel.
A cat-back stainless steel exhaust will run closer to $1000 for similar results (unless you're lucky enough to recycle a used CTS-V exhaust, of course).