I have 4mm o-rings, which reduce the travel about 1mm. At the moment they are in the closet, cause I dont particulary like them.
I tought that blues were too loud so I purchashed them. By the time they arrived I had already gotten used to the sound and feel. I definitely can understand if someone likes them, it gives them somewhat more "quality" feel. As far as the typing speed comes, i do not find myself typing faster with them. The 1mm travel reduce is insignificant in real world usage, atleast in my opinion.
If you want to get o-rings for the blues, i'd recommed for you to search the 1mm thick ones. The travel after the actuation point feels very natural, and who knows. maybe you will type faster with them, once you get used to them.
The 0.4mm reduce will probably be just fine, I just think that the effect is too small with such a small travel reduce.
But you should definitely try some of them, there isn't much to loose.