Just so we have everything accessable..
Channel – Charged Skill
Characters can summon forth the energy around them and fling it at a target, thereby causing damage. The Empath can throw a packet for "1 Magic" per second charged, up to a maximum of 15. If you have this skill twice, you can charge up to 30. If this skill is charged for greater than fifteen seconds, then the Empath may not move after charging.
Skill Mastery – Transmutation Effect
This potion lets you realize your true potential. While imbibed it reduces the time it takes to use any Charge Skill by 5 seconds (to a minimum of 2 seconds). It also lets you reduce the time it takes to use a Timed Skill by 1 Minute (to a minimum of 1 Minute). This lasts until the end of the current Combat/Scene.
So this is what I see -
Channel is a charged skill, so you reduce the time by 5 seconds (minimum 2 seconds), soo..
1 magic = 2 seconds (why would you do this? you just increased the time by 1 second)
2 magic = 2 seconds (no change whether you have skill mastery or not)
3 magic = 2 seconds (it was reduced by 1 second)
4 magic = 2 seconds (it was reduced by 2 seconds)
5 magic = 2 seconds (it was reduced by 3 seconds)
6 magic = 2 seconds (it was reduced by 4 seconds)
7 magic = 2 seconds (it was reduced by 5 seconds, which is the maximum you can reduce it)
8 magic = 3 seconds
9 magic = 4 seconds
10 magic = 5 seconds
11 magic = 6 seconds
12 magic = 7 seconds
13 magic = 8 seconds
14 magic = 9 seconds
15 magic = 10 seconds
And so on...
I would think that you are correct how you posted it. I'm curious what the other response has been and the reasoning behind it.
Skill mastery states it works on
any charged skill. Channel is a charged skill. Therefore, Skill Mastery works on the Channel skill.
(disclaimer - I'm not a GM, so I don't get final say.)