The earpiece volume does seem lower then on my iphone 5. I just picked up two iphone 6s and they are both the same.
The tiny speaker (earpiece) is different then the speaker coming out the bottom of the phone. That is the one used for the speaker phone, music, videos etc. They are two completely different components. There is no problem with that one for me.
I am assuming this can be fixed with a software update. If enough people complain. As everyone knows you really have to crank up your speakers on any stereo before they blow. They probably have a software limiter on there.
That is is of course assuming the limitation is the software and not that there is not enough wattage (power) going to the earpiece, if that is the problem then that would be a hardware issue.
My suggestion is that as many people call into applecare as possible. So that they are very aware it's an issue. I'm sure sure how much effect posting on the forum has for long-term change. This is the only way they will upgrade the software in the next update. And if it's it a hardware issue, they might fix it with the next iPhone 6 model and it will come under a warranty repair.
But they are selling so many phones at this time of year, this probably isn't a high priority, so hopefully in 2015 they will move it up the list.
Just my thoughts.