CES have fitted a few R34's with systems (owners here on the forums). All I can say in terms of the difference between a very high flowing loud system and a CES System is volume by a lot on the outside and pretty much no drone on the inside. The performance when considering relatively stock cars is negligble.
I mean if R34GTX's 7 month year old baby can sleep the whole way through a cruise up and down hills, I'd say its quiet.
On that note though, if you are planning serious mods, you may want to consider a more expensive or freer flowing system like the more expensive jap brands OR, get a cheap high flowing system with whatever tips you want and get an Apexi Exhaust Controller Valve (electronic costs a bit more than the lever operated one). That way you can have an easily switchable system. Not 100% on the legality though.
Some R34 GTT or R34 GTX-T's came with an exhaust volume control (only 2 or 3 settings though) from the factory.