Actually now that I think of it, there may be one advantage to the RLV R2 pipe over the R4 when it comes to sound. The tip of the R2 exhaust is angled downward while the end of the R4 extends straight out. The exhaust pointed downward can help alleviate the DB readings when being tested.
No serious, top-level competitor runs the R2 in stock moto however. The R4 definitely has a peakier top-end than the R2, even though the R2 is more forgiving to drive. Some silencers have a curved or adjustable tip to them that you can move around to point the end tip straight down, (my silencer has a curved tip that can be pointed downwards, which helps greatly). The right silencer and an airbox (with the foam filter attached inside it), should be enough to pass the sound test, regardless of pipe used.