The z's with t-top require extra structural reinforcement that is under the car, to make it more rigid. The slicktop does not need these extra reinforcements because the roof is solid. 2nd reason they are light. They only came with cloth manual seats. Power seats are heavy! My tt is a goog bit heavier then my slicktop with the extra structual reinforcements, the hicas, the extra hardware, (turbo's and intercoolers etc).. and power seets... but you'd never know it from all the extra power my tt makes over my na. The difference between the two cars is undescribable.