I think leadfinger is on the right path here.
You are assuming that all the springs and all the internals were manufactured exactly how they were intended following their calculations and expected energy results to the millionth decimal point.
That is often not the case and springs vary widely across Microtech Knives. I do not believe that Microtech makes their own springs so I am sure some spring have different constants than others. A small change between two springs makes a big difference on your thumb.
Stretching the spring will not ruin its temper as no heat will be applied to the material.
The purpose of stretching the spring is to exceed its elastic limit so that when it compresses after manually stretching it, it will compress to a longer spring with additional space between each coil. The spring does NOT stay in its stretched out state! The stretching of the spring during normal operation of the button does not stretch the spring close to where it was manually stretched; so no risk of breakage will occur.
The spring will still act as a spring yet the half a millimeter or less that it stretches will reduce its potential energy and make it easier to operate.
For reference of the above, you can look in any Mechanics of Materials textbook for Hooke's Law and Stress-Strain graphs.
Granted stretching the spring is an extremely CRUDE way of fixing this problem. However, if the spring is stiff, cleaning around the spring will not change it.
You also run the risk of breaking the spring if you stretch it beyond its tensile strength.
I always recommend cleaning a knife out thoroughly prior to attempting such a drastic modification.
Ideally, if you are able to send the spring into a company who actually manufactures springs and ask them to make the same sized spring but a little weaker; that would be a great alternative.