Hi Rene, in short...the guest will not see any of the storage efficiency savings; you can't claim that space back for the file system to use.
In your thin provisioned environment (both LUN and volume), the storage efficiency savings are at the storage layer only and release blocks back to the aggregate to allow you to create additional volumes/LUNs. The space savings for block based (SAN) storage is on the storage, unlike file based (NAS) storage that passes those savings to the clients. To realise storage efficiency savings directly on the hosts you will need to use NFS datastores.
You will need to increase the size of the RDM LUN on the storage and make that available to the Windows server to expand the file system.
Again, just to point out that since you've thin provisioned both the volume and LUN, you only need to monitor the space of the volume (and aggregate) as well as the file system. You do not need to worry about the LUN itself being 100% full. It is also good practice to regularly run (out of hours) SnapDrive Space Reclaimer (only supported on physical RDMs) or enable the space_alloc feature on the LUN to release those deleted blocks.
Hope this helps.
Thanks,
Grant.