I wouldn't be so sure. Given that this is a new backend for GNAT and the first one is close to 25 years old I assume it will have more modern optimizations and experience behind it - presumably leading to something faster.
Also, I was referring to AdaCore's implementation of the compiler backend. GNAT's architecture is structured like so:
GNAT Frontend (parsing, analysis, expansion) -> GNAT Backend (translation of frontend tree to something GNU or LLVM can understand) -> LLVM or GCC
This backend is quite complex and has a large role in how LLVM can optimize. My argument is that GNAT's GCC backend is 25 years old and a new backend written for LLVM probably generates better/cleaner code.
Also of note is that the GCC backend is written and C and the LLVM is actually written in Ada meaning its more uniform with the GNAT frontend.