Even the "DIY" version may be more that some kiddies should mess with.
First, even the most cursory glance at the sites should tell one that these kits are more than just a "replacement gasket" or "a replacement of the stock silicone groument seals and gaskets (A minor rebuild)".
Anybody who thinks that's all a kit is, should probably not be touching anything mechanical, especially power tools. And definitely not my car.
The work for most trans kits requires skill. That, and competency. You screw this up and your trans is toast. You can do this in without removing the trans, but you need to keep the process very clean. Dirt or grit will be a real problem.
Besides gaskets (usually one or two that remap trans fluid paths) there are new springs with different rates, valves, rods, and balls. Not all of these are used for every installation.
They're used to regulate pressure to the vacuum modulator and shift valving. By increasing the pressure required to initiate a shift, the slipping is reduced. By reducing the duration of the shift, heat is reduced. Both of these modifications will increase the firmness of the shift. This may be a mild or hard increase. Most Taurii would likely not benefit with the Steet n Strip levels of shifting. There would be no real gain in performance with 200HP motors (275HP w/CAI) and a hard shifting car is more annoying than the constant din of loud exhaust.
I had a B&M shift kit put in my old Z/28 with the mid-level modifications, just below SnS. At first the shifts were very noticeable (compared to the soft, mushy factory settings) but in a week, I didn't even notice it. (Note: 1st and 2nd gear loss of traction under hard acceleration was a pleasant reminder of money well spent.
The one slight danger of a well installed kit might be the fact that these can and often do eliminate lower gear lockouts. As one who used compression braking, there is a risk of throwing the trans into a lower gear at RPM it wasn't designed to handle. That could be a bad thing.
I'm not sure if I'd even want the most mild upgrade in my SEL. I really like the seamless shifting. But I would like to decrease the throttle kickdown time. I guess that means chipping the car. But I don't want to risk the warranty. I also miss the ability of being able so manually select 2nd gear. Waiting for the kickdown from Drive (not OD) is a drag.
(Oh, and for the one or two who didn't get it, the 275HP w/CAI thing was a joke.)