Build for multiple speed grades

Please read my previous post. Do not add any migration devices. There is no need to do that, and it is not the correct way. All you have to do is to make sure that multi corner static timing analysis is being performed. Depending on your device and Quartus version, this is the default anyway, so you might not need to do anything but just checking.

Multi-corner TA means that, at least, the fast model is used as well (besides the slow one). The fast model, besides considering best case voltage and temperature, it also consider best case silicon process, which is exactly the fastest speed grade. This is regardless of the speed grade of the selected device. Fast model timing is the same for all speed grades. And once again, the reason is that there is no guarantee that a specific part isn't faster than the specs. The only guarantee by the manufacturer, is that it is not slower than the marked speed grade. In other words, you can purchase C8 parts and they might perform as C7 ones.

Bottom line. Perform multi-corner timing analysis. And not just in this case, but always.