Placing your mh so high it is as good as not using even though you will be lowering it down ... The optimun height is 6-8 inch above water surface which is quite close and if you raise higher you will lose the light intensity which is not advisable...
As for T5, there are High Output tubes and Normal Output tube FYI... The ones most of us using here are High Output which is 24w, the dymax ones are Normal Output which is 21w, there are even lower one which you can find at most diy shopz..
Hope you know that you will need to drip the kalk and not to pour in during water change...