There is this $700 spring design software that I use:
http://www.smihq.org/public/software/asd6.html
All of the extra power extractor springs that are popular - I bought them and put them into the software - and it flags them as outside of good engineering practice.
The original M16 spring, however, was clearly designed by someone who knew what they were doing, as the software says it is great and has stress within normal limits.
The problem is, carbines need more extractor force than the M16, so the original M16 spring is not quite good enough. There are two ways to solve it - use a stiffer spring (but then they are over-stressed because, given the tiny cavity they sit in, wire of a desirable thickness will no longer fit for these springs with more force) - or use an o-ring with the original spring.
The o-ring is the correct solution, and not a gimmick - as it is the only solution where you get more extractor force without a spring design that is over 45 percent of the material's tensile strength.
We went one step further, and made the original spring out of higher grade wire - Rocket Wire, but did it without making it stiffer, so the higher grade material serves simply to make it last even longer.