I'd say it would depend on the surface along which they are moving. A flat smooth surface may be problematic for prehensile feet as while instinctively trying to grab, the foot would tend to curl, pushing off more than a flatter foot might. However, if there are rungs, grooves, or other surfaces suitable for grasping, prehensile feet would not only be faster, but also more versatile. For instance, without them you may be forced to use both arms to pull yourself along, but with them, you can move along with just your feet leaving your hands free. Alternatively, you could use hands and feet to move and could very well move much faster and with much more control than a typical human-like biomorph. Just don't challenge an octomorph to a race and you should be fine. I don't ever take rpg rules as gospel, inflexible and rigid. I consider them more as guidelines, suggestions, and best practices. If a rule doesn't make sense to me, I just change it. If a situation conflicts with a rule, I adjust it for that situation. House rules, as they are called, allow you to run the game as you envision it.