several warnings using altlvds

I would have to try, i used it in a CycloneII C6 at 300 MHz. I just tried that and got it running at 350MHz on a EP3C10F256C7 device, the C8 complains about the minimum clock width.

A C6 device is always faster then a C7 even at the 'slow' edge. That is the ultimate distinction. As on the 'fast' edge all speed grades may come close to each other.

TimeQuest will properly constrain your design according to the speed grade specified.

But if you can make it run for a C8 it will also run on a C7 or C6 speed grade.