Looks like I have done a lot of the research you are looking for.
The tall boy mx5plus bar is about 75mm further forward as was previously suggested, and that will see you loose at least two to three inches of rearward seat travel. Im 185cm- far to fat to be in an mx5 really,


What has not been mentioned is that mx5plus have already designed and built a cams approved high bar. When I combined this with my Cobra Imola (W I D E edition with stripped foams) on the standard na runners I was 50mm or more below the bar and felt ready to race. I then screwed sail channel along the rear of the windscreen between the screen and the roof rubber and made myself a 'bikini' top skin that threaded into the sail channel and strapped back to the frankenstein bolts with d's. With the addition of a shaped PVC centre bar that was a simple press fit,and a velcroed flap to wrap around this sewed in, I had/ have a roof that is near silent at freeway speeds and a bar height that is cams approved. The rear parcel shelf will still get wet if it rains, but for a non daily - this is a great solution.
The torque tube in the plus bars are great for stiffening the chassis too btw...
Short story - Danny at plus is the answer here- as long as you have reached the point where you are ready to lose the roof.
I am in the process of selling my NA as I have upgraded to a turbo, so I may well be selling the mx5plus cam bar shortly... I have a spare bikini skin if anyone wants to buy it too...
Hope this clarifies some...
Cheers, Rich.