does anyone have any opinions on the floor cross bar versus a rear tower bar (probably MazdaSpeed). I like that it would not take up space in the trunk, but if benefit is greatly reduced...

 

You are talking about two different things here. The Autoexe floor crossbar is for stiffing the unibody. Movement of the rear shock/strut towers causes changes in the alignment. A rear tower bar is for keeping this from happening during hard cornering.
I don't think the floor crossbar is neccessary on this car. A rear tower bar could show some improvement, but is is way down on my to-do list.