I suggest what it will do is:
- ensure you stay dryer and warmer than someone running almost naked, assuming you are intelligent enough to put it on
- and so help you stay rationale in the navigational decisions that you may have to make
- and help you run rather than stumble and shuffle along, cold, shivering and depressed and possibly hypothermic.
The most important aspect of surviving in a fell race in extremis is to stay on the route and to do that you need to be calm and thinking rationally, and that is a lot easier if you are warm and dry. The five classic deaths from hypothermia in fell races in England + Wales were all off-route when found dead. (And so was Chris Smith, the international mountain runner, who died last October in Perthshire).
On the spectrum of fell runner attitudes: at one extreme there are the majority who believe that nothing bad will happen to them because it never has; and then there are those who never quibble about carrying kit because they have experienced having to ponder if this was their last day alive.