The IMR is still better than the ARX. It's good that the ARX has an increased magazine size but the IMR reloads so quickly if you cancel the reload animation. Both guns are a 3-4-5-shot kill at different ranges. The ARX has a better range, but the IMR shoots faster so has better TTK at the majority of ranges. The increased fire rate of the IMR is what makes it better than the ARX.
The Impact is the best IMR just because it's never less than a 4-shot kill. If you want a silencer, then the Hushed is a good choice too, as it only loses hipfire accuracy for a free silencer. Ordinarily, I wouldn't recommend the IMR Heart Breaker, but I sometimes use a parabolic mic in SND. However, it drops off to a 6-shot kill at range. You should avoid using the Feedback and the Pillager because there's a typo in their code which greatly reduces their range and makes them bad. After them 2 variants, the Boar Strike is actually the worst IMR. It isn't worth reducing the fire rate.
The Hole Puncher is easily the best ARX, and it's the only one that competes with the IMR's. It has the joint-best 3-shot kill range of any burst weapon along with the Steel Bite, but doesn't lose fire rate like the Steel Bite does. The IMR's are better than the Hole Puncher at close range because of the fire rate increase. The Hole Puncher is better than the IMR's at mid-range because of the great 3-shot kill range. They're similar to each other at long ranges, except the Impact which is the best burst weapon in the game at long ranges. The Steel Bite is the second-best ARX but it's in a distant second-place. It isn't worth using the other ARX variants that lose fire rate, meaning the Red Eye Coyote, the Damnation, and the Head Shot.
I think you should use the IMR unless you have the Hole Puncher. Use any except the Boar Strike, Pillager, and Feedback.
I'd rank the burst-fire assault rifles something like this:
The top 3 are close. Don't use 5-8.
Here's my spreadsheet that I use to compare the guns.