Season completed using V11 and using Simulate Result (no match plan)
Mate, when I play with "Instant Result" I'm still overachieving but my results are about 30% worse than when I play in normal way due to these major reasons:
1) My assistant manager(in general any assistant manger behaves the same way) very rarely substitute DR(CWB) position in the matches due its constant high ratings during the matches and it means that 1/3 of every match you play 10 man because after 65 min of every match the "Condition" of CWB position drops less than 70% and he starts just walk on pitch and make a ton of mistake with his every move but this position is crucial for the attack and the defense in this tactic.
2) The assistant manger never uses the Waste Time tactic and it also causes another significant hit to results because smart of the Waste Time tactic might improve your result up 30% in the season.
Right now you are doing great becasue you play in the low leagues where everything is 10 times easier than in the top divisions of major world leagues such as English Premier League or LIGA BBVA, every time when you go "up" it becomes much harder to overachieve, especially, if you play with "Instant Result".
Every league in the game has "Reputation" and "Rank" and this "Reputation" and "Ranking" can be seen in the game, all that were made not just for nothing, it was supposed to give FM players some understanding about of the level of difficulty in this competition.
So any testing in the low reputation leagues are useless because you can dominate them with the default tactics that build into the game and the game shows its real difficulty only when you play in the most challenging competitions in the game (vs the best AI managers and the best players).