According to my calculations, the higher your haste is, the WORSE slam gets in a limited rage scenario. If you're swinging at a 2.5speed, slamming once between white hits sets your swing timer to 3.0speed, which slows your rage generation 16.67% (two slams slows your rage generation 29.6%).
Compare this to swinging at 3.0 speed originally. One slam slows your rage generation 14.3%, and two slams slows it 25%.
Another way to look at it is that slam doesn't just cost 15 rage, it costs 15 rage PLUS the rage you're losing from delaying your swing timer. The more haste you have, the more rage your slam "costs" you, because delaying your swing timer is worse at high haste than it is at low haste.
My take on it, then, is that if you want to go this spec, you're WORSE off by splamming, and better off keeping a more regular rotation (or at the very least, keeping MS on cooldown)