The P8 maximum length is 170. Why they don't make a 175 is anyone's guess.
I ski on the RX8 in 170. 6' 2" ~ 190 lbs. Solid Level 7. I downgraded my self-assesment from 8 to 7, when the Head Raptors taught me I am not as good in bumps as I thought I was.
The only gripe I ever had with the RX8 was it's performance in crud and chop. It just gets banged around too much for my liking there. I know technique has a lot to do with it, but to be honest, I would rather be on a ski that makes it easier when in this environment than work on technique with the RX8. I am not talking Western cut-up powder but the heavy Eastern slop that gets pushed around into piles by lunch time. This can be a little unnerving on the RX8 when moving at a fair clip. The shovel gets deflected fairly easily, usually when I least expect it. Being in good balance helps, obviously, but it's not fun to get distracted like that -- which tends to happen a lot in crud on the RX8.
In crud that is sketchy, that's when I bring out the IM77, but I have grown weary of this ski due to it's general lack of spunkiness. It serves the purpose but is not the most exciting ski and is quite damp. I am actually selling them to my cousin. I dislike the all-mountain midfats in general. All the ones I have tried will get the job done but lack excitement on the groomers. For me, they are "bring them out when you have no choice."