It's +3.5 damage per hit that always works.
If I remember correctly under the old system the equivalent would have been 2d12 to 2d14? (Assuming you had the Monk past life feat). If you had the feat and a +die step item it would have been 2d14 to 2d16 I believe.
The difference is easy to calculate. The average number for a die to land on is calculated with the following formula (where m is the number of sides on the die):
For a d6, it is easy to calculate as an example: (1+2+3+4+5+6)/6=3.5
For the difference between 2d14 and 2d12, we take 2*(1+2+3+...+13+14)/14 - 2*(1+2+3+...+11+12)/12=2.
Doing the calculation with 2d16 and 2d14 also yields a value of 2.
So my rebuttal is that in the new system, this feat is better than it would have been in the old system (as die step increases have been changed into +(base damage), since 3.5 > 2.
So to whether Improved Martial Arts is better or worse than Precision, I think that Improved Martial Arts is still better, though this one is debatable I will admit.
I still think that Improved Sunder is a worthwhile feat (it gives much of the benefits of Precision, except to the entire party), and it requires Power Attack. If you take Precision also then, then either Power Attack or Precision is a wasted feat. Additionally, most of the damage from a Monk is base damage, not dependent on critical hits (plus many monsters don't have much fort). Improved Martial Arts increases your damage in every situation, whereas Precision only increases it if you have to-hit issues or are fighting extremely high fortification mobs. Because of that, I believe that Improved Martial Arts gives the most benefit.
As far as I know, the answer to both questions in yes.