Having just added Luna Pro to the Signature bundle after intense agonising about the added value, I’m going to say it was worth it for me.
There seems to be a synergy between all the Luna Pro stuff, some but not all of which you already have in Spark. It feels greater than the sum of its parts. Maybe it’s a feel or workflow thing but everything just fits together beautifully and easily both in sound and use.
I think the value is in this synergy more than in the nitty gritty of what licenses are added, and for non-DSP users the bundle is presently the only way to get it all (I think!). There are more licenses per £ in the native bundles.
As for the nitty gritty though, for non-DSP users with Signature (or I think Spark) Luna Pro adds:
1. ATR-102 as mentioned above, beautifully integrated into the Luna console.
2. API Summing in Luna (hard and messy to reproduce with plugins)
3. Ability to use the API channel and bus comp as the Luna Console.
Hope this helps