For every 1/10 he saves at the start, he saves double that at the other end. Those 3-5 krpm stall converters are loose. If they launch just above idle they get a bigger tq hit. But if they launch at 1-2 kprm, the suspension is pre-loaded better, so it's a matter of experimenting. They say the max they could even launch at is aprx 2.2 krpm, the brakes will not hold any more than that. Loose, high rpm 2:1 ratio stall converters is something I have no experience with. And it all works on the street as well for DD use per the folks that have them.
If he didn't use the 5 krpm converter, and used the oem 1.5 krpm converter, with it's 1:1 ratio, the car would feel like a dog, as the NA coyote is making dick for Tq down between idle and 1.5 krpm. Takes too much time to get it up into the power band.
High stall converters are also used on the 3V auto cars. But toss in a blower, centri or PD, or nitrous, and it all has to be very carefully configured and optimized. I burn the tires off in 1st gear, but it throws me into the back seat in 2nd gear... up to 60 mph.... which is what I wanted for typ around town street use. 3rd gear it pulls a little less up to 90 mph, then into 4th, where the pull drops a bunch, but still pulls pretty good up to 110- 120 mph, then it's a slog up to 150 mph @ 6 krpm. I have seen plenty of higher tq 05-10 cars, where their 3rd gear pulls like my 2nd gear. Then the problems start up with wheel spin in 1-2-3, then back pedaling. You can only do so much with a street car, with typ street tires. I wanted some mileage from mine, and have to use it in the wet, so no DR's. DR's are not the best for cornering, which I don't do a lot of..except for hwy on ramps. I have seen 550+ rwhp Roush cars where they would almost turn sideways in 3rd gear on the hwy, the back end was all over the map.
Local prez of mustang club traded his 08 GT-500 in on a new at the time 14 GT-500. He can't get any traction in 1st + 2nd if he tried. They are > 600 rwhp out of the box, new. The 285 rears that come with them are not up to the task.