Re: 6 speed 4L80E
All overdrives are weak because it needs a small gear driving a big gear. Automatic transmission OD is no different. OD is not designed for power. The final OD is designed for high speed, low rpm highway operation to cruise at a much lower rpm to save fuel. Splitting the gears with an OD still needs a small gear driving a big gear. In an automotive transmission, you just won't get a big enough small gear to take any abuse.
In a highway tractor, it's a little different. It's common for a big truck to have an 18 speed transmission but it doesn't really have 18 gears. It's really just a 5 speed with a high and low range for 10 gears plus an OD that can split all the gears to give a total of 18 gear ratios. The OD gears are in the back of the transmission and are separate from the main gears. They use 2 countershafts to increase the load strength on the output shaft. the only time you need to split a gear is when you need to change the engine rpm one or two hundred rpm. Running empty, you can start in low gear and go through to high in only 10 gears or less, not needing the OD gears at all.
The 4L80E 6 speed conversion is the same thing. The split gears is only needed when in a truck as a tow vehicle and you need to drop down half a gear when climbing a hill to keep the engine in a slightly higher rpm range until the rpms drop down again for another 1/2 gear split. In a lighter vehicle, the extra gears are not required because it doesn't need as much HP to move the lighter vehicle. Climbing a hill, it's unlikely it won't even drop out of high gear. When I towed my car through the mountains years ago with my 454SS pickup which has a 4L80E, the transmission never dropped out of OD at all going up the mountains.
A proper 6 speed tranny would be more like the 6L80E with a very low first gear. It would give good acceleration off the line with something like 3.23 gears in the diff and have an OD for high gear on the highway.
Trouble with most 6 speeds transmissions is they're not really a 6 speed. The T56 has the gear ratios of a 4 speed tranny and has 2 OD gears. Unless you cruise down the highway a lot, you'll probably never use the OD gears.