If you want to own DM up, the most important thing, beside l2p, is spec all the high DPS buffing talents. So lets look at the DPS gained from the arguable talents (raid buffed DPS, using Tier 3 gears. Numbers roughly got from Chalon's current DPS calculators):

Adreline Rush (Direct-scaling):

Assuming you are doing 1400 expected/average backstabs, AR is 150 energy, or 150 / 60 * 1400 = 3500 damage, over 300 seconds, or a mere 11.7 DPS per talent point.

Murder (Direct-caling):

Assuming you are doing 750 DPS, murder adds straight 15 DPS when it applies, or 7.5 DPS per talent point.

Ruthlessness (Semi-scaling):

Assuming you are using SnD only, every 30 seconds, the actual CP/s = 0.6 / 30 = 0.02 CP/s

SnD DPS is around 110 of the overall 750 DPS, so each CP is worth 110/5 * 30s = 22 DPS/CP * 30s = 660 D/CP

0.02 CP/s * 660 D/CP = 13.2 DPS, or 4.4 DPS per talent point

Relentless Strikes (Direct-scaling):

Assuming you are using SnD only, every 30 seconds, so its an extra 25 energy every 30 seconds.

25/60 * 1400 = 583 damage, over 30 seconds = 19.4 DPS per talent point

Lethality (Semi-scaling):

Assuming 40% Crit, and Backstab DPS assumed to be 233. Working backwards, Backstab DPS is 122 ( 0.3 * 122 + 0.7 * 122 * 2.3 = 233).

So Lethality = 122 * 0.3 = 36.6 DPS per 5 talent points, or 7.3 DPS per talent point

Improved Poison (Non-scaling):

Assuming no VP, 15% Hit, 2.0 speed MH and OH, Backstabbing rogue.

20% * 130 * (90% * 1 + 95% / 4) = 29.575 ~ 30 DPS per 20% proc rate.

30 DPS per 20% proc rate => 3 DPS per 2% proc rate, or 3 DPS per talent point

Conclusion:

- (Assasination Tier 2) 3/3 SnD is absolutely needed for combat daggers. Murder produces better DPS/talent point compare to Ruthlessness, so 2/2 Murder.
- (Assasination Tier 3) Lethality is 7.3 DPS per talent point vs. Relentless Strikes' 19.4 DPS per talent point. 1/1 Relentless Strikes + 4/5 Lethality.
- (Assasination Tier 3, Combat Tier 7) AR is 11.7 DPS/talent point, but would require 3 talent points investment (2 fillers), so its actually 11.7 DPS/ 3 talent points. The other choice is 1/5 Lethality @ 7.3 DPS + 2/3 Ruthlessness @ 4.4 DPS each, netting you 16.1 DPS/ 3 talent points.

http://www.worldofwarcraft.com/info/classes/rogues2/talents.html?005223105000000005325000055010020005000000000000000

Overall efficiency: 2/2 Imp SS, 3/3 Imp Gouge, 2/2 Imp Sprint as fillers (total of 7). The rest are all DPS producing talents, vs. 15/31/5 build with total of 9 fillers.

Hope this helps. Feel free to correct my errors.