I went all the way to level twelve constantly winning promotion , won the league 4 times but never finishing worse than 5th with one of my teams. I only ever had 90% teams to start a season so I guess that would be about average , so yeah without much effort I won 4 leagues in 12 seasons.
As for your predicament I would guess that you are correct inasmuch as you will be placed with similar rated teams and so would your mate. You invested to make a 120% team so you get placed with similar , your mate the same , he would go against other mediocre teams , even though you both won the league first up. That is my understanding of it. Is it worth investing early ? In my opinion , not so much.