We already have access to all the T3 gear from the mine.
here is what the final upgrade will get us:
Another 6% discount to all dilithium requirements for fleet projects!
Another 6% discount for all dilithium stores (dilithium store, rep stores, potentially crafting stores??)
Rich Dilithium Mining Daily Mission (possibly a small upgrade project)
Dilithium Tailor and Costume unlock (I think is is another small upgrade project)
Interior Second Level Access (just visual/cosmetic upgrade)
Purple Quality Mining Doffs