The 6 is a super hot plug, going to a 5 unless you are going to be using it in 40 degree or more of colder tempratures would not be recomended. A hotter plug needs less gas to run right, and that makes it even hotter.
Believe me, Honda has a huge R&D department and they fully test everything they sell. Use what is recomended, unless you are putting together an hybrid of mixed models Redneck style.
Remember.... You can rarely in the new engines out guess their engineers.