There are several potential points of failure for what you're trying to do: having the right key code, sending the right thing, and having the program accept the sent keys.
My guess is that you've already correctly determined your Joy8 and whatnot.
In terms of sending the right thing I have found that some places respond better to {Ctrl Down}k{Ctrl Up} than to the methods you've listed, so you might try that and see if it works.
Finally some games do a pretty good job of stopping Autohotkey from interacting with them, in which case it may be extremely difficult to do. You may have already looked at this AHK in games guide but if you haven't it may be useful.
Also in terms of troubleshooting, does a new tab open with Joy8::Send, ^t in a web browser?
Staggering inputs may also help:
Joy8::
Send, {Ctrl Down}
Sleep, 5
Send, {k down}
Sleep, 5
Send, {k up}
Sleep, 5
Send, {Ctrl up}