Hi,
I live in Newcastle Australia, there's a bike shop here that does billiant mods to bikes and I have talked and riden with a guy with an 05 C90 who got him to modify his bike.
The shop opened up the air box, K&N filter, powercomander, debaffled the pipes and remapped his bike. They got a 12% increase in power and 15% more torque.
His bike now sounds much deeper but not loud.
I am going to get them to do the same to mine shortly ... when I have saved the cash up.