Hello @Macrafin,
as FedeNow mentioned you can see what are the possible Max Stats that you can get in the upgrade window , however there are the extra substats you get from rare and epic gear or the extra roll on one of the Substats if the Gear is a legendary Piece. Also you will gain Experience from rollin enough Gear so you will eventually learn all the Rolls on 5 stars and 6 stars gear and later on the difference between low roll and high roll.
I hope that was helpful.
Alpha H