Yes, It is woth it to be topless in a 99 Cobra as the car can be made just as stiff if not stiffer than a hard top.
Here is what I did to my 2001 Cobra convertible to stiffin it up.
I welded in myself a Griggs convertible roll bar that attaches to the critical B-pillers and rear bulk heads. I would stay away from the roll bars that bolt in to the non critcal paper thin floor pan as this does not add chassis stiffness and my punch through the floor pan in a roll over.
Griggs weld in convertible roll bar kit
. In my opinion, this is the best SN95 vert roll bar avalable.
The other key set of parts to making a SN95 stiffer than hard top is to add sub frame connectors, but here again, just plain old standard sub frames are not enough for the SN95 convertible chassis.
The solution is a frame kit. Here I am installing a Griggs upper frame kit with bulk head stiffners that are welded in the middle to the Global west lower subframe connectors that were already in the car. With the weld in roll bar and frame kit, my 01 Cobra vert feels stiffer than a hard top.
At first, My Cobra had the Global west lower sub frames and later on, I welded in the upper half of the Griggs frame kit. The Griggs Frame kit is a very labor intensive install as basically the whole interior needs to come out, the floor pan section right above the front factory frame needs to get cut out along with part of the boxed section of the floor pan to make room for the frame rail that is sunk into the floor pan. Bulk head stiffeners located below the rear seat are also welded in. Finally the upper and lower subframes are joined together with a thick gusset. Thankfully, I did al of the cutting and welding my self so I didn't have to pay for the 8-10 hours of shop labor required to install the Griggs Frame kit.
The Griggs set up is simply the stiffest set up available and with it,my convertible feels stiffer than a hard top.
One very fast 98 Cobra convertible took third overall at a hillclimb