I bumped it up to 51 before this last game. You may be right about it making the CPU pass defense worse, but I felt that before my players were dropping too many passes. The two games where I completed only 11/28 (and 12/28) passes there were a lot of passes that my receivers dropped. All of my main targets have 90+ catching and catching in traffic so more often than not, they should be able to hold onto the ball through contact.
This game looks bad as far as Kaeperknick throwing for 450 yards, but I generally abandoned the run game early as GB went up 10-0 on me pretty quickly. I threw the ball a lot as you can see, almost 50 times and completed 63% of those passes which is about right, maybe a bit high. Also I have changed my system a bit from a vertical passing game to a more horizontal game which is the reason for the higher accuracy and number of passes. For now I'll keep the slider at 51 and just lower my User Pass accuracy until I get it about right.
I suppose the fumble slider is an inverse slider too? I really wish EA would tell you which way increased/decreased the chance of events occurring.