You can play with a longer silicone tube for a longer length before resorting to cutting up what you already have. With a real pipe, the diameter of the outlet makes a difference too, and the length or the stinger a bit too, if it makes it restrictive. You can see if you are in the ballpark, by blowing into the stinger like a flute, and your finger over the entry point of the header. (not trying to be kinky here) Compare the musical note to the rpm coming from the motor. 13,000 rpm is equal to about an "A" You are not too far off with either pipe, and the stubby one may unload a couple thousand rpm in the air and be "on" the pipe better than the longer skinnier one. A smaller prop load could bring it up to be on the pipe too. You have already surpassed the open faced rpm so are much farther ahead than a stock muffler with either pipe.