Flexi Joints are usually used for cutting down / dampening the vibration from the exhaust and engine; they don't change the sound that comes out the back!
In my Nb race car I run a slipper joint exhaust system for ease of removal and that still vibrates like the proverbial with 3 engine steadies on the motor, when I was running it all clamped up the 3 into 1 headers that I was running cracked down at the collector!
The picture below is a picture of headers and exhaust system I did for a clubman, that runs a slipper and spring style joints on the exhaust system with variable length primaries to tune to optimum power, The motor is solidly mounted in the chassis with front and rear plates, and with vibration from that it still has a habit of getting a leak if there is not enough silicon in the joint at the collectors!
This is the old system!
It went from 45.8kw at the wheels to 67kw at the wheels and we still have to play with the primary lengths! At is first meeting with the new pipe work the owner droped 2 seconds a lap with a cracked inlet manifold!
Just because you have an engine steady or two does not mean you are going to cut down vibration through the exhaust that can come from the motor as well at the resonation from the exhaust its self!
If you can afford to spend a little more I would get a flexi Joint fitted!