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hippy
5Nm on a properly lubricated M5 bolt is about 5000N bolt tension. You don't want any plastic in that load path. If it doesn't explode, it will creep, and if doesn't creep it will be insufficiently stiff and then you've effectively got a spring under the bolt head which compresses and loosens the grip on the bars every time the load increases. Even if the bars don't slip, the pressure on the unloaded side drops to zero and the bar/stem interface frets, creating the initiation point for a fatigue crack.
M5×0.8 is very close in size to the #10-32TPI Tom Lipton tests here:
https://www.youtube.com/watch?v=QC6QciSh6gw