wearing knee pads wont let you take more risks on fast descents as they don’t offer enough protection , your mind allows you to go faster as your skill level increases . I ride a lot of fast xc like yourself and yes I do actually wear knee pads but only on new trails that ive never ridden before or if I know I am going to be doing some nasty rock gardens , and they do start to rub after 30 miles irrelevant of what ever pad you are using