There is a probable answer to surging during warm up - it is related to the fact that for emissions there is a requirement to get the cat up to temperature as quickly as possible. This is achieved, especially under light load by retarding the ignition periodically on one or more cylinders so as not to loose drive completely. This is a technique that Rover use and others (my old Alfa used to do the same). It is noticed particularly on light throttle openings when the mixture is lean - under larger openings there is more heat in the exhaust gas so the retardation is not applied. This is important as the manufacturers understand that there may be occasions when some drive is needed, so to have nobbled the power completely would be a major issue.
The fact that replacement ECU's do not show this problem is that they are less concerned with warming the cat up quickly, but this could also contribute to a reduced life as a rich mixture when the cat is cold will decrease the life of the cat.
I am sorry this does not answer the surging issue on more recent cars when hot, this sounds like a mapping issue, but I doubt the warm up issue would be an issue Subaru would fix as they have to pass emissions.

 

Interesting...thanks . My engine surges when hot and its not so recent....MY94.