If however you are pressing the + key and keep clicking after the "Game update rate changed to 8x" message, you will get the confirmation click but no further messages. Same thing if you click - after getting the 1x message.
I think this is intentional, since it's always between 1x (normal) and 8x, can't go above or below, so that's why it doesn't display any messages since you're not actually changing it.
I'm still unclear though exactly what it means: " Game update rate changed.." I understand your reference to replay (like a sports replay in slow motion) Perhaps that is what these Keys are for to change the Replay speed of a recorded game?
I thought, being an RTS game, perhaps gamespeed referred to an overall slowing down of the games so old geezers like myself don't have to click on things with the lightning speed of youth
"Game update rate" is their fancy way of saying how fast stuff will go. Unfortunately, beyond playing at the slowest settings (customized when you create the game, by clicking the Game Options button on the launch screen), you can't slow it down any more. You can speed it up to the 8x, so everything will happen 8x as fast (just watch some ships move around in 8x 
), and you can slow it back down to normal.
But it's not exclusive to watching replays, it does modify the actual play speed.
