Ah the old high pressure shooter topic. Back in the 40d days we had a ton of veterans always spouting off about how pressurized water doesn't push things around- that hasn't changed has it?
If you want to make sure the water drifts about one pump per five to seven tile wide opening should output the right volume of water. May need to delay it slightly so that the initial water in the cistern can make it's way out though.
*Pokes head in*
Yeah, sorry.
Pressurized water doesn't push things around. At all. Read this. It's still accurate and relevant.
The theory here I believe is that when you flood a dry corridor with a pump, the leading wave of the water is never a solid 7/7 and as it moves forward, would carry creatures with it.
I can comment on the lethality of 2/7 slowly moving water with a 2 z-level drop.
Also, I've gotten terrible luck with my world I've genned to test this on. I've taken my time setting up the trap, but 5 years into my fort and I've seen mostly kobolds, a few snatchers, and only a single ambush party from the goblins. I plan to implement the water-down-an-entrance-ramp idea later, for now I'm putting the finishing touches on a large reservoir with a 2 wide path horseshoeing around it. I made the mistake of making a fortification in a place I didn't intend, and when i tested it the first time, water has been trickling out of the hole down into my drainage. I thought this wouldn't be much of a problem, but I didn't have the presence of mind to raise all the bridges at once to equalize the interior water level. I've had 4 dwarves path by the trickle of water and get knocked off by 2/7 flowing water. Its funny though, the place they fell is only a 2-z level fall, while the entire rest of the trap is planned to be a 4 level fall. Each one has shattered at least 2 limbs, and one even died on impact. This trap should be plenty leathal. After falling 4 z levels onto grates, the center of the horseshoe will contain a pillbox for my marksdwarves, who will then be able to pick off the stunned and crippled invaders at their lesiure. The pillbox has a gap so they should be safe from all but elite bowgoblins that haven't broken their hands yet. In the event of something nasty, they can evacuate the pillbox, and the entire pit area/chamber can be completely submerged for drowning. Drainage will be handled by pumps with protection from building destroyers.
Now if only they'd send another ambush or siege or something.
Also, when my miners were falling down the last 2 ramps to get to the bottom of the trap before crawling out, at one point the water caused them to collide, further wounding one of them. This makes me think the 'water down a series of 6-15 ramps and then a pit' would be completely destructive.