Its my opinion that some people resort to the restriction partly because it's in the kit and because they hear some people say it works. It's easy to install too if after building your trans you get overlap.

And it's easy to understand for those people - those that look at the trans clutch/band application charts that see on the 2-3 the front band is coming off and the front clutch is coming on. So they theorize that since the band is coming off - in order to have an overlap it must be the front clutch causing the problem by coming on too soon.

They feel something has to be coming on too quick to cause the overlap so conventional wisdom says slow that application down. Hence the restrictor.

I correct the flare in someone else's trans by installing the matched package of front clutch release springs (these have a direct impact on the 2-3 apply even though many call them release springs as do I) clutch pack clearance (minimal impact from this if within the span of correct specs), the front servo spring package and the operating lever ratio. Many of the guys doing transmissions do not understand hydraulically what is actually happening on the 2/3 shift.

Yes, they know what's on and what's off and the hand off on the gearchange but hydraulically they don't understand.

Not sure if this is a good analogy - your car has a terrible stumble on WOT at low speeds - it's got an AVS which actually has a loose air valve spring. Do you adjust the spring or choke the engine down with a small air cleaner (restriction) and drive down air flow?