My gicleur (love that word) requires at least 8mm opening into the spout, with a fairly flush 15mm diameter total clearance around the opening.
Restricting flow anywhere in the spout decreases pressure and volume of the flow. Restricting at the intake gives off a nice smooth flow at the tip without messing with the designed pouring spout.
I have a few Hario's and it is not completely necessary to have a sniper kettle to manage the flow, but it can be much more enjoyable and offer a lot more control even on a Kone brewing a liter with a 60g bed.