I can attest to first hand knowledge of the CTS-V pulling away from an E-39 M5 up to about 125.

To be honest, I wanted an M5, but couldn't find one, so I got the V. I was afraid to run into an angry M5 on the road some day, for fear that I would be disappointed in the V. Though I still pine for a M5, I'm not afraid of them performance-wise anymore.

As far as mods go, for under 10k, I can add a Corsa exhaust and a magnacharger to my car. Similar mods, with a custom tune, are putting down 475 hp to the wheels on CTS-V's. Scary power.