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I installed a set of the frame stiffeners a couple of years ago and I can definitely say they made a noticeable reduction in cowl shake and steering vibration when driving with the targa top removed. The first time I drove my car with the top removed I thought there was something seriously wrong because the handling was so different. Installing the bars helped a lot but it did not cure the problem 100%. The improvement was enough that I can drive the car without the top and not be worried about how it feel. Do a search here on the V2V frame stiffener bars and you will find tons of information, probably more than you'll want. As far as the need to drill holes, I do remember drilling two holes in order to install the rear attaching plates for the bars. It was not a big deal. I can also tell you that the bars were nicely made and fit well.
No modifications required for cars with the mounting holes for the convertible x-brace. There is a bolt hole at the rear mount to add an extra connection to the rear control arm bracket. An extra hole needs to be drilled in the control arm bracket though.
I also installed them. Instructions were pretty easy to figure out. Had to drill one hole on each side in the control arm bracket. Noticeable difference after in stall.