I've used pads on elbows infrequently, and ALWAYS in preference to a thicker elbow. The reasons for this are more practical rather than technical:

1 - The heavier elbow is an out of spec item. Especially with modern 3D CAD based modeling, the systems correctly make is difficult to put in an out of spec component. When it drops on the ISO, there's a good chance that the fabricator is going to miss the fact that it is different.

2 - You cannot walk up to that elbow in the field and see that it is the heavier wall that you asked for. You will have to get a specific UT reading taken to verify.

3 - If you call for a repad, that becomes a separate line item in the Bill of Material (BOM) that stands out on it's own. It's much more likely to actually get noticed by the fabricator and it will be obvious to anyone who walks down the line during construction whether or not the pad was installed.

As my mentor has told me - you can't just specified something so that it can be understood, you have to specify it so that it cannot be misunderstood.