I doubt it would be noticeable in that case. Once you start talking about a large number of different strings, you get into the realm where memory dominates computation speeds. You might shave off a few clock cycles in your computation, but your overall processing time is still going to look pretty much identical to memory used divided by memory bandwidth.
I imagine the most performance relevant use of toLower is a case insesitive compare over a large container or database of records. That could be noticable, even if each string is short.
That's a good point, you're probably right.