HDMI video processing from HDMI in

I doubt it.

If the company who designed that product had anything about them they would ensure they shipped with the cheapest FPGA possible. So, I have to assume that they determined that the EP4CE30F23C6N was a) appropriately big for their logic requirements; and b) a suitable speed grade.

The EP4CE6F17C8N part you're suggesting is much smaller (able to implement far less logic) and slower.

You're also looking to add an Ethernet interface, which will require additional logic as compared to the Color3 box.

The best option is to code it, compile it and use Quartus to help you chose an appropriate part.

Regards,

Alex