Join Date: Nov 1999
Location: Where the Navy sends me...
I had a modified MM 6pt in my '89 convertible. (It's been sitting in my garage for a while, as I pulled it out to do some interior refurb.) As castine917 said, based on the jacking the car up with and without the rollbar installed, it definitely improves chassis stiffness - and that was just with it bolted in.
Ancient picture during initial install:
We decided to just weld the door bars in (instead of leaving them as swing-out), and also added a diagonal in the main hoop. Welding the door bars in was a mistake - just made it a pain in the ass to climb in and out of the car for normal, around town driving.
The guy doing the install welded some 1x2" box sections below the floor where the main hoop and door bars landed. You can see the 1x2" here between the MM full-length SFCs and the convertible rocker sill reinforcements:
'89 GT convertible, N/A 357W, MM front susp w/425# coil-overs and Koni D/As, MM TA/PB rear setup (Koni Yellows, 390-430# springs), Stoptech 332mm / 12" Baers, MM 6 point rollbar, Maier 1.5" flared fenders & quarters, 18x9.5" front/18x10.5" rear Enkei RPF-1s (street), 18x10" CCW C-10s (track), etc...