My take on the question is a little more limited than Ben N's, and we differ on your specific question. Take some examples of question focus:
- Making existing hardware work the way it was intended: clearly on topic
- Building a system or modifying a system by combining off-the-shelf components (and not a shopping question): I can't readily think of anything that would be off-topic
- Repairing a system or device using commercially available replacement components that are intended to be user-replaceable: probably on-topic
- Designing or building (prototyping), your own device from scratch: generally off-topic
- Re-engineering a device by modifying its circuitry or mechanical design: generally off-topic
- Tweaking a device to improve its performance or characteristics in ways that don't involve substantive changes to the original design (things that wouldn't void a warranty): probably on topic
The kind of change you described in your question would be an example of the last bullet. The issue I had was more with the focus of the question.
If it asked something like, "Is there a way to limit key travel without re-engineering the keyboard", a solution might be sticking 4mm thick o-rings under the keycaps. However, the question asked about the difference between using 1.5mm+2.5mm o-rings vs. 4mm o-rings and the clarification on what kind of difference you anticipated talked about the physics of the o-rings and the user's experience.
I thought this focused the issue on mechanical design and material science, which to me, is outside the site's scope. The effect on the user is something that could be determined only by experimentation. So unless somebody already tried exactly the same combination of o-ring sizes and material (a long shot, especially since you didn't mention the planned material), it would attract guesses.
This question was a tough call for me, a grey area. If the community responses here indicate that the majority of users consider it on-topic, I'll retract my close vote.
There was also a question of whether you were simply asking whether a 1.5mm thick o-ring plus a 2.5mm thick o-ring were the same total thickness as a 4mm thick o-ring. If that was the intent of the question, that isn't really a computer hardware question.