Sorry don't agree with your first point. You'd need to take out a lot more than 2 rows of seats. The tapered profile of the carriages makes the existing overhead racks way too small.
It's not the profile, it's bad design. The racks in First Class are plenty large enough; that type could be fitted throughout.
Also moving luggage racks around doesn't fix the fundamental issue of the door layout on the bodyshell essentially being designed as a high density commuter unit.
Door position prejudice makes it into the thread again. You remember Mk1s and early Mk2s had end doors
and a centre door?
Also on the gearbox, if you take out the 4/5 speed gearbox on a car with a small engine (say 1.0L) and replace it with a 6 speed it's hardly going to make a difference in acceleration or performance. The issue is the engine does not have enough horsepower or torque to move the mass of the vehicle quickly enough. I can't see how changing a gearbox is going to change that.
As built they have hydraulic transmissions with a lock-up facility (and
may have two gears in lock-up, I forget which units do and which don't). The problem with this is that they waste a load of power on heat and noise in the transmission fluid. If that's difficult to understand, consider the idea of driving your car by starting off in third slipping the clutch - you won't start off quickly because you're just wasting energy heating up the clutch plates.
The Class 172 has a more traditional mechanical gearbox which uses a gear appropriate to starting off, and can therefore accelerate much more quickly from a standing start as it doesn't waste energy.
Consider, similarly, why cars don't tend to offer a hydraulic-coupled automatic gearbox on the smallest engine option...
