The 2006 XJR that I just bought had something similar done at some point in its history. Intermediate mufflers were removed with straight pipes added and rear resonators replaced with Magnaflow mufflers.

I am toying with the idea of adding an X-Pipe or H-Pipe. What is the benefit of the X-Pipe?

You certainly can't "fly under the radar" like a stock XJR.

The sound reminds me of a 70's muscle car. It is loud, but not unbearably so.

 

Does anyone know if this causes problems with California smog certification? It doesn't need with the cats, so if guess not, but I want to be sure...

 

Since your mods are downstream of the CATs, it should not matter. If you removed any secondary CATs, I would expect you to have an emission compliance problem since I think CARB runs a sniffer up the exhaust to test as part of inspection.