Presumably if you have the FPGA with native HDMI out to a modern tv it probably wouldn't have perceived input lag. Maybe a frame or two but nothing that you could really feel.
What I am really hoping to see out of this project is a MVS and AES DUAL slot board. The ultimate all in 1. I would throw down money for that. Add in a bios chip slot so we can easily upgrade (and support) the real unibios, and that would be great. I know that is a bit of a pipedream though.