WingoWango,
There is a lot of research indicating that pilots under stress revert to their early training instinctively. All that capacity built up flying single pilot will crop back up and possibly interfere as the SPA trained pilot fights the urge to go it alone and not utilize the other half of his team. I imagine this tendency will reduce over time and be gone by 2000 hours or so if the SPA trained pilot flies in a very disciplined multi-crew environment. If they fly in a cowboy environment all bets are off as to whether they ever lose the go it alone, maverick tendencies.
Capacity is largely a function of intelligence, reflexes, perception through the five senses (I shouldn't discount the sixth one either), speed of thought processing, etc. It can also be degraded by aging effects, alcohol, fatigue, obesity, smoking, lack of exercise, poor diet, etc. The age degradation is usually compensated for by wisdom and experience.
Capacity therefore is mostly innate and can be degraded by poor lifestyle decisions. Some people have it an some people don't. It can be improved if one has enough to start with. We may never know if one training method improves it more than the other and we surely won't if MPL is never implemented.