Saber Guy Hi,
ali here 3demon is my home computer name . My Current hawk with a AMT Mercury, Two tail servos, tail former mod, Tail plate glassed in and Pilot weighs in at 11 kg with no fuel so your target weight is pretty spot on.
As for Tailpipe. The only reason why I changed mine was that I know from previous experience that the high eflux of the AMT Turbine lends itself to a different tailpipe than the Jet cat style constant diameter pipes. I had no problems at all with he normal Skymaster pipe when I used the P-120. The weight saving was a big advantage (Approx 1/3 rd of the weight) But the custom tailpipe was a lot more than 1/3 rd more expensive