Whirlwind does 85% weapon damage for 20 rage on each target.
Slam does 275% weapon damage to the main target, and (0.35 * 275%) = 96.25% damage to each additional target within 2 yards, for 25 rage.
Damage per rage:
Whirlwind: 85/20 = 4.25% weapon damage per rage
Slam: 275%/25 = 11% weapon damage per rage on a single target.
The rage efficiency of additional adds for slam cleave is:
96.25%/25 = 3.85% weapon damage per rage
Don't forget that sweeping strikes duplicates the main strike:
0.50 * 2.75 = 1.375
0.50 * 0.85 = 0.425
Final formula:
Slam: (412.5% + (Targets hit by the cleave * 96.25%)) / 25
WW: (42.5% + (Targets hit * 85%)) / 20
Here's a nice little excel spreadsheet showing rage efficiency and damage done per gcd:
http://gyazo.com/fb332466f1ab03756b356fcaf42c5469
The big deal for slam's superiority (And why Whirlwind should be removed from your bar) is the damage dealt/rage spent per GCD. If we're strictly looking at rage efficiency, whirlwind can be a better choice if it will hit 3+ more targets than Slam cleave would.
If mortal strike/colossus smash/thunder clap are used on cooldown, which they should be for the most part, they will consume 40%~ of available GCDs. This tier, thanks to 2pcT16, arms rage income is kind of ridiculous. It's around 12-13 rage per second.
The sweeping strikes glyph negates most of the rage cost of re-applying sweeping strikes itself, so that's going to be ignored for this.
A GCD is 1.5 seconds, which means 12.5*1.5=18.75 rage per gcd
Slam/Whirlwind are only going to be used **AT MOST** on 60% of GCDs, thunder clap is 10 rage and everything else is free/grants rage.
Rage spent per GCD * 0.6:
25 * 0.6 = 15 Rage
20 * 0.6 = 12 Rage
(Both are lower than the average rage income, leading to lost rage/wasted rage on Cleave/Heroic Strike)
Using whirlwind pushes 6 rage into the "Lost/Cleave/Heroic Strike" bucket. Slam only puts 3 rage in that same bucket, which means that when they are compared against each other, Whirlwind actually has a rage cost of 23.
If I plug in a 23 rage whirlwind into that same spreadsheet:
http://gyazo.com/c981101288000fc49512984bc97f1f69
The rage efficiency gains are no longer there when Whirlwind hits more targets. Whirlwind now needs to hit 4+ more targets than slam cleave to be worthwhile rage efficiency-wise.
All of this is not even including that most AoE is of the bursty-nature, and if rage is not a concern, then the damage done per global is the most important factor.
Which leads to my final point, and why there are 2 charts on the pictures above.
If ONLY looking at damage done per global, whirlwind needs to hit 5 more targets than Slam cleave to be worthwhile, and if colossus smash is up on the main target, this breakpoint is increased to 6+.
(There are some points where 3/4+ targets is the breakpoint, but the gains are trivial.)
As far as the range concerns this tier:
Heroic Immerseus: Adds are stacked up, and die before bladestorm is over.
Protectors: Most raids either stack them up so that slam cleave will hit all targets, or spread them out far enough that whirlwind isn't going to hit them.
Norushen: Single-target rotation with sweeping strikes up.
Sha of Pride: The aoe-adds (can't remember name) are always stacked up nicely for slam cleave, but also die before bladestorm is over.
Galakras: One tank has all adds, 90% of them will get hit by slam cleave.
General: 1/2 target fight.
Spoils: One tank has all adds, same as galakras. The only time that it may be iffy is when Mogu-side spawns the stun-dudes (I'm great at names), but slam cleave will hit them the first 2 globals that they are up as they spawn on the boss itself.
Siegecrafter: Depending on your strat, mines will spawn on top of the boss itself which will get hit by slam cleave, or they spawn far away and range dps kill them.
Paragons: Why are you padding.
Garrosh: Adds are ALWAYS stacked up on top of the boss. Slam cleave all day.
It would take incredibly specific positioning of adds for whirlwind to hit the breakpoints mentioned above that would mean one of three things:
1: You're fighting TOGC 25 Anub.
2: Your raid is running a completely stupid strat for the fight.
3: You are slamming the red dot:
TL;DR: Much like the ability Cleave, remove whirlwind from your bars as Arms.
I do agree that the radius needs to be increased another 2 yards, though.