I was just thinking I should post this figure as well. As you can see, the regular SPP algorithm reduces the overall noise, while it is much more preserved in the frequency-warping version, as expected. You can see this particularly well in the 3-6kHz and 8-13kHz bands. Also, there appears to be slightly more base noise in general, probably due to increased phase-vocoder error. You can see this particularly at about 20kHz.