Thanks!  Most car questions aren't this complicated!!  I appreciate everyone's input.
I did get 
all of the OEM bracing replaced, albeit with much grumbling 

   The chassis isn't bad, but every bit of stiffness helps!
I know you can't beat the roll bar for stiffness.  You are incorrect, though, when you say, 
"if you can fit in your car with the top up you'll be lower than the roll bar".  Since the top of my head touches the soft top, it means the top of my head is 
even with the bar.  I sat in Tim's car with the M2 sport and was literally using the bar as a headrest.  I love the idea, but the reality, and the mechanics, aren't going to work for me (not without a 
lot more expense in changing seats, anyway).  I have a real concern with the head clearance issue since I've been rear-ended about a dozen times over the years.  The number of times I've rolled a car has been zero.  So while I'd 
like a roll bar, it has to be safe for 
me in the ever-present possibility/probability of a rear-ender!