Offset mode... nice! Thanks, Lynn...Boy, Sonar is deeeeep! Had no idea!  Wish my brain could keep all this stuff on tap...

Kev999 --- right, I'll add a Pre-master Bus, no fx at all, and use it just for the raising of the overall volume of all tracks routed to it, prior to it hitting the Master bus (which does have some FX on it).

Leave it as it is!!
 
-15dB is quite loud enough for a studio mix.
 
Your Mastering Engineer will love you.
 
My only caveat would be if this song is one of several songs for an album and the other songs are at roughly a similar but higher level, i might raise the lower one up a bit.

Funnily enough this track is "Part 2" of a 3 song set.  So, I'm trying to have it same levels as Part 1.    
I'm using Ozone 5 to master it --- so, only thing is, I'm having to push the Limiter threshold down quite a bit more than usual to get it simialr level to Part 1 - around 8db.   
Is getting the overall level more around -6db at mixing time, then use less limiting at mastering, going to give a better result than having the lower -15db level at mixing time, and doing more limiting?   Or will the result be anyway the same.   I have at least the impression that more liminting means more potential artifacts introduced ...?