Could you outline how exactly (using available components) higher performance could have been achieved with the previous size?
Better keyboard (the super-thin design is not as pleasant for most people to type on, key travel is important to touch-typists usually). Much longer battery life. We could have kept some of the ports we still want access to, and while that's not exactly "performance", it's certainly a feature that most of the people I know who buy super fancy laptops would like and would pay for. We could easily have gotten a 32GB RAM option, and there'd have been space for enough battery to feed it.
Or they could go back another size or so, and give us our gigabit ethernet ports back and things like that, and make a high-end machine which doesn't need its weight in dongles just to keep doing the stuff a couple-year-old machine could do.
Also, on the MBP of late, performance is usually capped at least partially by heat. A slightly larger machine could have had more heat sink, and more airflow, without needing larger or more powerful fans, and thus run cooler at a given workload. So that'd improve performance.
The "thin no matter the cost" thing is really hurting a lot of the things that used to make these machines great.