I know you were asking about T48 but In size and deployment style, the SB1000z is closer to the Tuba 45.
Comparing 1v1 the max output of the dual 18" is almost always gonna beat a single 12" even horn loaded.
However it does take a good bit of power to get there. A lot of shows I'm working on we are lucky to get 1-1/2 circuits to ourselves for audio/backline power. When you're sharing with Lighting and cattering and whoever else, haveing a rig that doesn't need a lot of power can be very beneficial!
Also the sb1000z are around 200lbs. I don't remember what the weight on our T48s are but I recently picked up a couple dual 18" subs from a club remodel and they are significantly more difficult to move around the shop by myself than the T48s..
I looked around the internet a little for reviews on the SB1000z and noticed a few mention that "while powerful these may not the most musical subs on the market" your mileage may vary.
Every speaker design is a exercise in compromise it mostly comes down to what features are most important to you and what features you can live without in pursuit of that goal.
Authorized Builder - Phoenix AZ.
Main Rig - 6 DR200s - 6 T48s (30")
SLA Pros for the smaller stuff.