I have a 23' Khaos 9R 155" and I love it, in my group there is a G4 165" and a 24' Khaos 9R 165" . I have road both several times as I am always thinking I need to go 165 but after I get back on my sled I always feel the most comfortable on my 155. In my mind it just feels lighter and easier to ride. I have never felt that I was held back by my sled, I have several times had to take my 155 up a climb to pull a ski for a 165 and it just gets the job done. The all over Mt. type riding you described above is exactly what we do, I love the creeks, sidehills and being out of my comfort zone. as far as being tired I try and stay pretty physically fit I don't feel that the sled wears me out at all, (no electric start either). For me I am very happy with my 155, I may have to carry a touch more speed in certain situations but I enjoy that aspect. I personally have not experienced enough of a benefit of the 165 to make me switch. again this is just my opinion and personal experience. I am 54 years old been going out West for 20 years, mainly ride out of Afton Wy. Hopefully I was able to provide some info. I feel like I just started to ramble LOL.