I don't think so because that your saying meaning, very early in the morning of Sunday you have a full team (of 22 players) plus 3-4 unsigned players, right ?Hi! Quick question: if you buy the players, but don't sign them, they still count for your top 14 players?! You can do taht if all your positions are full...
So at 04.00 am, the program counts the 14 best players you already have and making the draw of the Cup. It doesn't matter after that if you buy or sell your players.
Another thing is that if you buy those 5* players this season (and having them unsigned), next season they will be 4* so this will give you a disadvantage to confront your higher level (and quality) opponents.
The key to this method is to have 14 low quality players (or a mixed team with 7* and 1-2*) as can be, till Sunday 03.00 and after that to enforce your team with nordgens or scouters or * q4/q9 veterans.