There are very few hardfp benchmarks that really compare just this feature and leave the rest of the system identical. E.g. comparing Debian armv5 with floating

There are very few hardfp benchmarks that really compare just this feature and leave the rest of the system identical. E.g. comparing Debian armv5 with floating point emulation vs Ubuntu armv7hf of course yields a huge gain, but that mainly is not due to the fact that floating point values are now passed in fp registers. If SW relies heavily on passing a handful of floating point values in function calls, it should be rewritten to use NEON/SIMD on ARM - and then all you pass is a pointer to the data, regardless of the calling convention. And once you move to writing the mission critical stuff in assembly, you can chose your own calling convention for subroutines as you see fit anyway.