Thanks for considering XenGallery as a replacement for the XenMediaGallery.
- For the import, it should be OK as it benefits from the Import system of XF. Therefore there is no issue with huge number of records.
- I cannot give the comparison in the querying between XenGallery and XMG since I do not dig into it yet. But I am sure XenGallery serve faster than XMG with multiple thumbnails solution. And the photos are directly served to users in a unique filename system to prevent users accessing not-allowed-photos. Compare to XenForo which leave this to the attachment handler which need more queries to checking the permission, route, output the files... This is my most positive point compared to XMG. I do not have a record for a site who has over than 2 million images yet (they usually not tell me btw), but there are some customers who imported directly from vBB, PhotoPost with around 1 millions photos and they do not have any concern about the performance.
Of course I need to sell my addon, but in case you have to try before use because you have very big requirement, I can support you in this situation. If you see it cannot fit your requirement, just delete it.