Hi Cigar,
Your post sadly highlights one of the major inconsistencies with the 85% guidelines. The same model of car can have different MIS figures dependant on what was fitted at the factory.
This means that a low speck car will have a lighter MIS, yet if equipement is added by the dealer to match a higher speck car, the MIS dosen't chang because the parts were not fitted at the factory. So two cars that are to all intents an purpose are the same on the road could have different towing guidance when its based on the MIS.
I suggest it would be safe to base your towed weight calculation on any version of your car that has a higher MIS provided it has the same engine and transmission, and that your specific model has a high enough braked trailer weight limit.
85% of MIS is not cast in stone, and it has no legal significance.