Depending on where you place the two new subs, and your room, it could pretty much be as good as it gets on bass evenness. Gotta be better than one 15" regardless.
One thing, though. If you have the power available, you can build the boxes a lot smaller than the 1.2cu ft/driver quoted. One example is 0.7cu ft/driver for the 1260W, which gives Qtc=0.76 vs 0.59 for 1.2cu ft. Especially with REW and eq available, the difference in FR will be negligible. Real difference will be in power required to hit xmax at 20Hz, which is still reasonable at roughly 300W/driver vs. 150W/driver for the bigger box. Haven't checked, but likely similar story for 1262W.