Why would you even need tabbing?
I used (a working) F8 & /assist in XI for all my battle needs.
I know we have larger groups of monsters, so of course I'd tab for that but are you really having that big of issues? My biggest issue is "fight starts, I tab 300 monsters/players that aren't the group we're fighting, I target monster, it dies, I pull my weapon out and look like a jackass."
I'm way more concerned with that than I am sub-targeting during battle.