Skybreach is mostly fine, I find. Whether it’s too weak or inflexible relative to some of the (overtuned?) casters that were released later on, I couldn’t say.
Both this and Nightblade mastery support is something I’ve suggest myself, so I agree.
However, before adding a third mastery to a TSS set that already mostly works, please take a look at Allagast, which is in a pretty sorry state.