Chassis Stiffness Torsional Rigidity
 

I got the numbers for everything but 986 from youwheel.com....car-body-torsional-rigidity-a-comprehensive-list. 986 wasn't included but according the Excellence Magazine Boxster History page, the 987 is 4% stiffer. Therefore (987) 12600 Nm/deg divide by 1.04 gives 12115 Nm/deg for the 986. Not to bad really.
Still wish the roof wouldn't creak when I exit the driveway.

Lamborghini Countach 2600 Nm/deg
Honda S2000 7100
Mazda MX-5 (NC) 8132
Lotus Elise 10000
Porsche 986 Boxster 12115
Porsche 987 Boxster 12600
McLaren F1 13500
Ferrari 360 Modena 23000
Porsche Carrera GT 26000
Porsche 987 Cayman 31500

Perhaps you can draw the conclusion that if 13500 is good enough for the Mclaren F1 then the 986 is 90% awesome

I wonder if a hardtop changes it appreciably? I would think it would increase a bit, but probably not too much. Maybe the 4% to match a 987? Unless someone has somehow measured it, we'll never know for sure.

You are overlooking that fact that the 1997, 1998, and 1999 986 cars had a thinner main tub than the 2000 on cars did, which is why use of oversized (18, 19, and 20 inch) wheels often led to suspension mounts pulling out of the tub on the early cars, and which the later tubs were reinforced with thicker metal to prevent; so I would expect the 2000+ cars to be stiffer than the earlier versions.