There are multiple discussions about mage heavy teams from back when world PVP was an actual focus. Also other discussions on leveling more than 5 characters...

You can research those for more details if you want but my thoughts in summary are:

Pretty sure 4 Mages annihilate everything around them already so not sure going with 9 buys you much more. BGs are really the focus now and with no follow in BGs not sure it is worth it now.

Leveling any size more than 5 is slower than leveling 5. 4MP is likely the fastest 5man team to level. The only thing faster that 4MP would be a skilled Mage booster boosting 4 characters at a time (though you will lose some time swapping characters if you actually needed to do 5 characters to cap with a skilled booster it likely can still beat 4MP solo).

So in summary if your goal is 10 characters to cap your best approach is 2 5man groups.