We did it last night without any problems using our normal strategy, with a healer helping out on the last droid.
If you're facing the walker the droids we kill in order are 2 - 1 - 3. We find this order is helpful to avoid the AoE shots from the walker. We have one tank on droid 2 and one tank on droid 3 while the DPS take out droid 1. A healer helps interrupt droid 3 while the DPS move to droid 2. By the time we kill the third droid it usually has 3 stacks; even if just the tank was interrupting we wouldn't have hit 8.
Curious what your groups' dps output is and if you have actually done 8man HM this since the change.
Before the change I would agree 100% with your timing estimation. Since the change, the damage reduction on the bots greatly slows how fast they die if you do not get the majority of the interrupts. That was the first thing my group noticed... Why is the first bot taking so long to die...