Shifting at 4600 rpm?

I think only part of the story is being told here. If you have not rebuilt one of these things, I have a concern that the info given is only partial. I've built more than a couple but I don't believe I'm no guru with them. That being said... Kickdown linkage will not totally control the shifts, ever. If they were shifting that high either the main line pressure was jacked up using the regulator adjusting mechanism on the side of the valve body or ground the throttle pressure valve (done in the TF2 kits). The problem is when you only change one side of a hydraulic system beyond what it's capable of operating with, you create issues on the other side. I'll use the MP 5.0 "Hemi" kickdown lever as an example. Most good shops use the factory 3.9 or 4.2 lever. The 5.0:1 lever holds well, but doesnt release well.

High line pressure is a double edged sword. As far as "bonuses" to doing it:
It will increase the rpm at which the governor weights uncover the pressure port and the trans upshifts (kickdown pressure is a function of line pressure so if one goes up, the other does too by a proportionate amount... That's why they can make be made to shift higher..)
It increases holding power for all pivot points.
It speeds up all actions related to shifting. It decreases the time the kickdown band comes on (good thing for 1-2 shift where only the band applies, nothing releases)
As for what it does that is NOT good:
It creates more heat and parasitic loss from the pump working harder and can lead to the pump lugs shearing off especially in race sitations.
It greatly decreases the time it takes the high gear clutch to apply (bad thing for 2-3 upshift as the band has to release before the clutch applies and the band is much slower and is worse with the higher ratio band apply lever).

Most shift kits contain methods to help both sides of the hydraulic circuit and these should be carefully followed. Simply cranking line pressure and pulling the accumulator spring isn't the best way to do it. You're trans is better off left stock and unmodified. The shift kits are the best way to minimize any damage but because of the way the trans works manually shifting it will cause more wear than letting it shift itself with an "Auto" equipped valve body. The hard parts and passages are calibrated for the automatic function. By holding it in a gear you are asking the hard parts (like the piston return springs and shift valve passages) to work outside thier design parameter. It might not be very evident, and with care and experience it can be minimized, but it's happening none the less. If some have gotten things to work by only doing part of the work I'd say they got lucky, maybe more than a couple times, but it isn't supposed to work like that and I wouldn't call it good advice.