Does a lighter or stronger main spring reduce felt recoil?
An interesting question. The answer is yes and no. Physics class was a very long time ago for me and I don't have a mechanical engineering degree so I humbly accept any corrections to what I've written below.
Explanation of the NO answer
As with all springs, the recoil spring pushes in both directions at the same time. As it is compressed by the slide moving backwards, it is pushing forward against the spring plug and it is pushing backwards against the shooter's hand. This backward force is felt by the shooter and interpreted as felt recoil. Therefore, the stronger the recoil spring, the more force is felt by the shooter.
Explanation of the YES answer
The impact of slide against the frame abutment results in a sudden stop which causes the muzzle to flip upwards and is felt by the shooter. The less impact that occurs, the less force will be felt and the less muzzle flip there will be. Theoretically, a recoil spring could be manufactured with precisely enough strength to barely allow the slide to just kiss the frame abutment thus reducing the impact and muzzle flip to the lowest possible amount. However, there are so many other factors involved that this is a practical impossibility.
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If muzzle flip is the objection, there are compensators that will eliminate it.
If felt recoil is the objection, lighter bullets will reduce it as will the use of stronger mainsprings and flat bottom firing pin stops that will allow for the use of lighter recoil springs. These things will reduce the felt recoil but cannot eliminate it.
There are no perfect answers which is why we all continue to experiment to arrive at a solution that is best for us based upon a multitude of factors that are unique to our individual situations.
------ edited to actually address the question -----
I went off on a rant about recoil springs and didn't directly address the question.
Without changing anything other than the mainspring, a lighter mainspring will allow the slide to move faster to the rear while a heavier mainspring will slow the rearward velocity of the slide. The faster the slide moves, the greater the impact between the slide and the frame. The greater the impact, the more felt recoil and muzzle flip.