It likely wouldn't be 15% with an 8-wide x86 decoder if such a thing would even be possible within any reasonable clock budget. So in that sense a fixed width I

It likely wouldn't be 15% with an 8-wide x86 decoder if such a thing would even be possible within any reasonable clock budget. So in that sense a fixed width ISA does buy something. Also, given that chips today are mostly power limited, 15% power usage is 15% that could be used to increase performance in other ways.