Theoretically Devastate is good dps. But as a dps tool for pve, it's rather high threat.
Since your getting more dps white for d/w, than a 2h weapon... Devastate should be more dps than slam. Slam also reduces your white damage dps, by adding 1.5 seconds to white damage attacks (minimum, I don't know many people who can be 'perfect' about auto attacks and slam).
devastate is on a 1.5 second cooldown, is easily spammable, and functions well with a d/w dps set anyhow. both slow MH damage and higher AP values affect it greatly, just like they do for WW or HS.
Without imp slam, flurry, imp zerker and rampage, I'm laying my bet that Devastate dps is superior to Slams dps.
On a level 73 mob, which means glancing hits and higher dodge rates...
Without flurry, imp slam, or a weapon spec, 143 hit rating (9%), 2381 ap, 30% crit, 120dps 3.6 speed weapon, with a 200 MS latency for slam timings; slam should be around 400 dps with white damage. If you slam after every white hit. Which you should have enough rage per hit to do with those stats. You should even have enough to WW every once in a while, if you white crit.
Imp slam raises that DPS value by about 100.
With devastate on every cooldown with a 91 dps 2.6 MH and a 91 dps 2.6 speed OH, no flurry, 143 hit rating (9%), 2381 ap, 30% crit, 5/5 one handed weapon mastery, Defiance 3/3 (but not in defensive stance), it's like 700 dps with white damage.
But it's way higher threat.
These are super rounded numbers, I'm using a spreadsheet... So I don't think my math is exact, but it's fairly close. This is something I've been working on as a pet project, prot dps.