That's an interesting question b/c I never thought about how data is optimized internally. It would seem like CR can figure that out to some degree and process it more efficiently, but I doubt if it does so.

Yes, you'll have to specify the same data source for both the report and subreport. Re using the same formula, you can't do that. Everything is duplicated. However, if you use a record selection formula in the main report and then link the key data to the subreport, then this will restrict the data that is printed in the subreport. I'm not sure if that is what you are looking for or not, but it's about the only way to optimize the subreport to any degree.

Hmmm. I don't see how the drill-down feature replaces the subreport. Drill-down data lets you hide data and later expose it, but it doesn't group the data differently. The groups are all the same, just hidden. Thus, if you need different grouping categories, then subreports would be necessary.