Hi mattcase,
A thinner comfort layer would allow more of the firmness of the deeper layers to “come through” yes and you would sink in less than with a 3" layer but “how much” firmer it would be would would be very subjective and would depend on the specifics of the mattress (the firmness of each layer) and the body type, sleeping positions, and individual perceptions of each person. For some people this may be an improvement and for some it would be detrimental compared to a 3" top layer but for most people it would be noticeably “different” and feel firmer if all the other layers and components were the same.
You can see the details of their warranty here. Most defects in a mattress (which is the only thing a warranty covers) will show up in the early stages of the life of a mattress (generally in the first few years) so beyond that it’s much more important to make sure your mattress uses good quality and durable materials than it is to have a long warranty that doesn’t cover normal weak and tear or the softening and breakdown of lower quality materials that leads to the loss of comfort and support anyway. You can read more about warranties in post #174 here.
You would need to ask them about any exclusions that aren’t listed in the warranty but I know that most of the better smaller manufacturers (including SleepEz) tend to favor their customers in the “grey areas” of warranty coverage.
Phoenix