The point is that just because Goku didn't use ASSjin against Cell doesn't mean that MSSjin is stronger but ASSjin is a false form and a waste of time even if its stronger than SSjin. Still there's no evidence to suggest that MSSjin > ASSjin.
It's wouldn't be a waste of time in some scenarios. Grade 2 didn't stop Vegeta from making Semi Cell his bitch, who was at least on par or even stronger than his Super Saiyan form, same could be said for Goku against Cell. Goku could have easily just used grade 2 or even grade 3 when doing his warp Kamehameha too.
The grade forms are not a literal transformation like how SS2 is to SS1. They are the process of trying to power up while in a Super Saiyan state without training your body to acclimate to that power (conditioning yourself to hold more Ki). Grade 4/FPSSJ does just that but without the change in appearance and strain by having the SS form act as their base state, so nothing hinders them when powering up. So FPSSJ can use strength equivalent of that to the grade forms and beyond. That'S why I don't think grade forms can even be used after mastering Super Saiyan.