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I dont profess to be any kind of expert and have tried all of the above, on my commute, so no long 100km+ journeys.
C15 just didn't work and I think the top is too curved for me (others on here love it) and too narrow. Not sure about the C17 as I tried a short one and it was Ok but it not exactly butter on my butt.
The SMP does feel more comfortable and I think the reason is that once you are on your sitbones, everything else feels less pressured so you don't notice it as much. I also find that I can move around on it enough not to be uncomfortable and for short journey's is OK in just my mtb baggies.
I also had a forma before (the dynamic shape but no padding so similar to your carbon) - and that was uncomfortable for the first 5 mins then much better. At the moment the dynamic is nice to sit on, but I am still messing with my bike fit, so ask me once I get an inline post.
Okay, I have to accept that my Regale is doing me no favours in the downstairs department, which is a shame, because it works fairly nicely for my sitbones.
My ali-special carbon SMP is the right shape, but too hard for longer rides. Thinking of trialling a Cambium - can anyone (@edscoble ?) who's ridden both give me an inkling of how they compare to something like a Composit? Cutout vs none, C17 vs C15, etc.