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You don't really use the full range of movement when cycling/running so no, stretching doesn't make you more powerful. There is some evidence stretched muscles are less powerful.
Obviously a bit of mobility means you can comfortably get aero and aero = speed.
This is why you stretch for flexibility after riding, not before. You might do some warm up stretches before though.
I read something recently debunking the long-held view re. power and stretched muscles. @dancing james might have some thoughts on the latest thinking?
I read recently that flexibility isn't actually that beneficial for going fast on a bike, as long as you can get fairly aero, you don't need to be any more flexible.
In the running thread there was a link to an article about the sub two hour marathon that mentioned one of the ways Paula Radcliffe got faster was by getting less flexible and further reading suggests that applies even more to cycling.
(That's my excuse for not being able to touch my toes,.. It's so I'm faster, not because I'm fat)