Server API: calling GrantUserItems versus ModifyItemUsesRequest.

Well first, it's important to realize that very large inventories and large inventory grants can take a significant amount of time to complete. Adding a bunch of a single stackable Item ID will be quick, since the items are pre-aggregated before the work to update the player inventory though, so given that you're referring to modifying the item uses, you shouldn't see issues with this (it's just good to bear that in mind).

Some details for the specific scenario you describe:

• The grant call will take about the same amount of work in either case (due to the pre-aggregation I mentioned).
• Calls that operate on an item instance ID are extremely efficient, since that's their index in the table (no table scan required).

So it's actually fairly close, but for the extra Server API calls (which do increase the overall "cost" somewhat).

But the most important point is that by breaking the operation up into multiple calls, you're running the risk of the first call (or two or three) succeeding, but then the other calls failing due to the player losing connectivity. In that case, you're in a bad state. So I'd recommend making it one call to grant the items.