My favorite T5 tank overdrives them with IceCap. Ed's tank. Do you have good reflectors in the retro kit? If so, then I would do this and the pair of XHOs. I imagine that you are thinking the overdriven T5s for more output and the XHO for pop? Did the T5s even make a difference with the Radium running?
If you are worried about PPFD, then get that up to 10-11 hours. I would not be, though. Total Light Index (or something like that) is a Dana Riddle piece about lower PPFD for more hours is just as good as higher PPFD for less. Let your eyes be the judge. If you are getting good color and growth, then you are all good.