I would like to see if someone else can confirm this. I was messing around with the Tac 12 and I found that the time between shots could be slightly reduced by ADS while operating the pump. It may have been my imagination, but this is a forum for discussion. I didn't bother with the FP6 because it has lower ammo and so is slightly harder to test without equipment, but I assume it would apply to it as well.
I've thought this same thing with the L115. I feel like the RoF increases by going in and out of ADS. If I'm not mistaken, I think this is an intentional feature. Are you saying that while engaging in the act of aiming down sights the rechamber action is reduced or that rechambering while out of ADS is shorter than while ADS?
It wouldn't surprise me if the Tac 12 pumped a fraction of a second faster while ADS given that's where its advantage lies.
I haven't really tested this as I am currently trying to get the hip fire kills camo unlock. But it is something worthwhile finding out. Maybe Drift0r has done an in depth episode on the tac 12