Solution
The PCIe 1477 speed grade is -2
Regarding the impact of speed grade on multiple attributes, you can have a look at this Xilinx K7 datasheet . The answer to the above question depends on the design's as well as HW component's FMAX. Search using the keyword FMAX for maximum frequency for certain FPGA components in different speed grades.
The maximum clock rate is affected by the speed grade, but it depends much more on how the clock is used. Although 400 MHz might be possible in theory, in practice it's impossibly difficult. The logic can be run in the 150 to 200 MHz range (that's a much more practical speed), but it depends a lot on what the application and it's up to the code in a way that will run at that speed.