On Mac Safari accepts both shortcuts:

CTRL/CMD + SHIFT + T is used for reopening the most recently closed tab CTRL/CMD + Z is used for undoing the last user action

Since the shortcut for undoing the last action is pretty much universal I think it would be a good thing to have it also apply to the tabs in Opera.
CTRL + SHIFT + T would still be needed for when you want to reopen a tab after you did something else on the active page.
e.g. if you've typed something in a form CTRL + Z would undo the typing, while CTRL + SHIFT + T would reopen the last closed tab

Basically this would change nothing for people used to the CTRL + SHIFT + T shortcut, but would make it more intuitive for people that associate CTRL + Z with 'undo'