If you just take the inner cone off, the exhaust nozzle area will be too large. That's why the single piece cone is longer than the two piece, so it can taper down. The exit area should be ~ the same. The area of the two piece cone is equal to the outer area minus the inner cone area. If you can match this to the single cone area the thrust should be close or better with the two piece cone as it is more efficient. The reason the one piece cone has a deeper rumbling noise is because of more turbulence in the efflux. But, if they are matched and have the same exhaust temps, go with the one that works best or the one that sounds best