I have no experience with tempered nickel, but does it stay tempered after firing?
Regardless, with that in mind, that refinement drop does seem on the high side. I can't say whether it has to do with contact coils, personally I've had no problem using contact coils, spaced coils, or even partially contact, partially spaced coils since day one. I usually do not see that much refinement drop on any of them, but if the thing works great then use it!
Resist the urge to see issues that don't really exist.
Regardless, with that in mind, that refinement drop does seem on the high side. I can't say whether it has to do with contact coils, personally I've had no problem using contact coils, spaced coils, or even partially contact, partially spaced coils since day one. I usually do not see that much refinement drop on any of them, but if the thing works great then use it!
Resist the urge to see issues that don't really exist.