I would think that at least in the case of Honda, their blades and mower decks are designed and engineered to work together. I am sure if someone had all 7 or 8 blades that fit the HRS, HRR, HRN and HRX models you could certainly spot the differences. I reviewed manuals and replacement parts guides and the HRS is the only mower that I saw showing an optional blade (singe to double option). Blade length and hole pattern may allow you to mount any 21" Honda blade from current and past models to your HRN, but cutting performance may be negatively effected by changes in airflow. I am interested in how well the HRR baldes work for you.
I have change my HRS blades, but the optional blades for my mower are from the HRR216K8 and earlier models which are quadracut and not microcut. I have thought about trying just the bottom blade from the HRX as I primarily side discharge my clippings and I believe that this would give me the highest air velocity and disperse much better than the stock or quadracut blades. When mulching the quadracut would likely be best.
Speaking of blades, have you seen the blade used on EU spec HRX's?
I have also seen double blade setups for EU HRS's, but I assume that the pictured blade was designed for noise compliance?
At the end of the day, the HRX may always out-cut or out-mulch the HRN because of the larger engine and deck design. Either way, share your experience so that others might benefit.
Mike