It's not for efficiency, per se. As in C, to support arrays of structs all structs are padded so their size is a multiple of the strictest (largest) alignment of any of its members. If the example struct described above were 24 bytes instead of 32, then the second element of an array of those structs wouldn't be properly aligned for any of its uint64 or pointer members. On architectures that require aligned loads and stores that's not an efficiency optimization, but required for correctness.
What I mean is, reducing the size of a barely-used struct isn't worth the effort, but reduction of a key struct in a critical region, or a large bloated array of structs could be very useful.
So some measure of: possible reduction (which structslop provides), 'hotness', and total memory usage, would make this a killer feature.
From my bare-metal days we used to manually rearrange structs or use them packed, but it'd be great to have a tool that could attach to your codebase and do that analysis for you.
