I suspect that this is a combination of sprockets and diff exhaust. I had a NK mid on my RB but wouldnt pullin 4th. Tried a Jarspeed pipe with same sprocket set up, pulled in all gears but no power band...

tried a JL3 this was same as Jarspeed, smoother through the range but not as quick. then changed sprockets to 18 x 57 and stuck the NK back on now pulls in 4th and still has power band....happy day.
I would do as previos poster suggested and try the Scootrs pipe again with the new sprocket set up, you may be surprised..
syd