they might be hard pressed to match the Mongol empire on that

I disagree somewhat. There is not much verifiable data to support that imo.

Mongols were excellent in fighting, invading and empire business. They took slaves in the beginning, especially the craftsmen and women etc and took them back to Mongolia. Later on not so much - having slaves crumped their style, because of distances, until they came up with taxing their subjects/minions instead.