My FJR factory mufflers are very quiet. The thing you have to worry about with aftermarket slip ons is making sure you get the exhaust tips past the back end of the car so the sound is "outside" and behind the car, not under the car.
If you were trying to run all four cylinders through 1 slip-on you might have a problem, but with two I don't think nois would be an issue. If you get take-offs from a Yamaha FJR1300 or a Suzuki Hyabusa, you would have no problems with noise as the stock mufflers have to deal with noise emission standard, yet deal with 145+ horse power. They might even be too quiet.