I've found that 7-8 speed shifter/cassette combos work fine. They basically just added another sprocket on the back of the 7sp cassette, then adjusted the witdth of the freehub carrier to suit. Not a lot of development required to get the extra gear. 9-9 only though, same again for 10sp. Different index/sprocket spacing in order to get the extra gears.
Why not just give it a go as you've got the mech anyway.
I've found that 7-8 speed shifter/cassette combos work fine. They basically just added another sprocket on the back of the 7sp cassette, then adjusted the witdth of the freehub carrier to suit. Not a lot of development required to get the extra gear. 9-9 only though, same again for 10sp. Different index/sprocket spacing in order to get the extra gears.
Why not just give it a go as you've got the mech anyway.