Not much of a point since Apple ][ has the SWEET16 interpreter already as part of its BASIC ROM. But yes, it would be nice if the LLVM-MOS6502 folks put some effort towards feature parity with this toolchain. LLVM almost certainly has better auto-optimization than a new bespoke compiler.

According to Wikipedia the SWEET16 Interpreter is significantly slower than native assembly (Wiki says 1/10th the speed). A C compiler that targets 6502 would make sense if I want to code something to run on the Apple ][ that's relatively performant

Yep. Same chip: 6502. I'm guessing the nearest thing to this is probably cc65: https://cc65.github.io/