I've never seen any noticeable delay with the wireless keyboards ive seen being used, granted i'm not sure if the same would be true if you're sitting far on the TV. I don't think there will be a problem though.
wireless is just another barrier between you and the game. best use wired as they dont incur an performance hit, unlike a wireless keyboard that will need drivers. which take up system resorces. there is also a delay as the signal is sent...
then theres always the problem of running out of power just when you need it. nah wireless anything isnt a good idea for gamers.
I use a wireless mouse when I game but I'm 3ft from my pc.
MY HTPC are a different story and I can tell you that receiver placement is the key. If you plug it into the back of thepc you will have issues. The case is blocking the signal path. If you plug it into the front you may have issue still if theres anything metalic in the way. I used to have my htpc in the bedroom on the floor and the metal frame & springs in the matress would block the signal. I ended up rebuilding that pc into an old vcr housing which now sits on the dresser and doesn't look out of place next to the tv.