A friend order a full system exhaust for his 450X. the can he got says T-4R. I checked the PC webpage but there is not much info in there. I noticed that for the T4R you can get just the replacement can or mid pipe unlike the T4 where you only can get the headpipe or the can with the mid pipe toghether. Only difference I noticed is that the T4R can and mid pipe have only 1 retaining spring, while the standard T4 uses 2. So performancewise, whats the difference between the T4 and the T4R? .