There are some recent clips on youtube of an IS 350 that say "IS 350 with HKS exhaust" or something similar. I remember they were supposed to make a system originally for Lexus to be sold at dealerships but it disapeared about the time the cars became readily available.

Their website does not show anything being available currently.

 

I believe the clips you're referring to are some IS350 owners that have custom piping connected to universal HKS mufflers. From what I've heard the HKS system thru Lexus isn't going to be available anymore. Also supposedly HKS is just going to release a system themselves.