Of secondary importance, but yes - a longer sight radius will allow more accurate aiming (less aiming error), and as every bit helps in LR rifle shooting....
It was THE position used by the competition winners (and most others too, I might add) in BP LR target rifle - and it was actually popularized by the US shooters beating the Irish at Creedmoor at that famous match in 1874, even though it had been in use before... The popularity of the supine position grow rapidly after that. There were several variants of it, BTW - even 1 to suit wrist-mounted tang sights.
It is presently still allowed (and used by a few) for the Brit LR competitions, but only at long range. I presume the feeling is that at short range it would confer an unfair advantage to the shooter because of the added stability and longer sight radius.
