Is there any perceivable benefit (or, perhaps, disadvantage) to upsampling CDs--which typically are encoded at 44.1khz - to 48khz?
No, and in fact its a pretty bad idea unless you need 48k output for some reason, and even then I would recommend resampling after decoding, not before encoding.
Upsampling adds nothing but an increased storage space requirement. Since the normal reason for lossy encoding is to decrease bandwidth and storage requirements, upsampling could be said to be a "bad" idea or at least a not very bright idea.As far as effecting audio quality, it is probably a total null, but it is possible that the codec works better for 44.1kHz than for 48kHz. I don't recall reading anything about that, it probably isn't true, but having no interest in it, it is the sort of thing that might go in one ear and out the other for me.
...I do wonder if LAME would better handle preecho issues (and other rare problems) at a 48khz sample rate ...
What's with the accurate replay gain scanning? Are you using one of the tiny fraction of apps/plugins that actually makes use of RG info in the Lame header?
If you care about highest possible VBR quality you might consider using -V0+ or similar with my 3.99.5z variant.