You're still mixing clean oil with dirty oil, which is not good. Also, I'm sure many owners on here put their cars through alot during regular driving, and it's always best to play it safe and keep your oil clean with regular changes. Don't get me wrong, as long you have oil in there, you are unlikely to have any major issues...but with these cars and how hard they are driven, I think it's always better to lean on the side of caution.
I wouldn't take the chance of "half-azzing" it with such a sensitive and expensive vehicle. In one of those quarts you are adding might contain contaminates, like another member just posted. You just can't be too sure sometimes. If you are gunna do it, do it right. procedure. :tonquesmilie: