I am not very experienced with Arms having been a tank for my entire Wow career and only now working on Fury.
In a PvE setting, a 25% reduction in rage cost seems a little more valuable, and the savings should add up over the course of an encounter. More white swings turned into HS rage dumps. I don't know the math but I'm sure it could be done; rage generation calculated over time would show you over the course of a fight how often you would be able to que HS and with the rage savings, this would surely increase, assuming MS/WW/Slam whatever on a regular attack cycle.
As a raid tank I am constantly hammering HS/Dev so long as it doesn't effect my cycle. If with high end gear as an MS spec you're able to maintain HS spam with your cycle, then what good is 25% savings? I don't know because I don't know what rage generation is like for those guys. In my fury experiment, I have found I am hardly ever starved for rage but then, I am just working on mobs out in the world what are hitting back, not being tanked. But if you are jacking up a raid boss w/ a T6 weapon, 2.5K AP buffed and 30% crit, maybe you're swimming in rage already. /shrug
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)