Yeah honestly your 5Ts are already too close to the walls, so I think if you did go up to the 6Ts you would have to rethink your set up entirely.

Also, even though the 8D and the 6Ts have about the same frequency response, you can get a lot more low frequency output out of the 8D since you can boost the gain so much with its volume control.  

So if it were me I'd probably just stick with your current set up.