One thing that I've noticed over the last few years is that manufacturers are fitting brake light clusters that have a higher output - probably better reflectors and better quality lens design. The problem for us is that we get used to this and it gets easier to miss the lower output from the older brake light design when driving behind an older car.

Exactly -- brake light inflation syndrome. One of my pet peeves...

You can brighten the original lights considerably by spraying the reflectors with chrome paint, BTW. Really makes a difference compared to old, dark gray reflectors. And of course a third light is a great idea.