The proper mixture is 3 qts Redline D4 ATF + 1 qt Redline MTL 75W80 GL-4.
You should not use ATF straight regardless of brand; it's too slippery and doesn't support proper syncho engagement. You'll find that the ATF+MTL mixture is thicker and makes the synchros sluggish for the first 10-15 minutes at 30°F and below, but it will shift well. You'll feel the added drag until it warms up. At higher temperatures it's a significant upgrade--the notchiness will be reduced and your shifts will be faster and easier.
When shifting in anger at 7000 RPM, there's probably nothing better than D4 ATF+MTL. I have 10 years racing T-56 and TR-6060 transmissions with this mixture. The older enthusiasts I learned about this from have been using this much longer on the track and swear by it (e.g. 90's and 00's era Corvettes, Camaros, etc).
In case anyone asks, I am aware of the Tremec HP-MTF fluid but have not tried it. I believe it leverages the principles behind the D4 ATF + MTL mixture, but it is optimized for very high temperature operation. You can see that in the viscosity specs--I would not use this on a daily driven car (it's thicker than straight MTL):
One problem with running thicker oil on a daily driver is that it's slow to get up to temperature; it's not until oil thins out that it can get into all the crevices and micro-textured surfaces and really take on heat. That's why you see the oil temp climbing slowly on your car for the first 50 degrees; the next 50 degrees takes half as long.
The optimal ratio of ATF to MTL changes as the transmission gets older and the synchros wear out; very worn T-56 and TR-6060s (e.g. 250k miles or a lot of hard launches) require a 50:50 ratio of ATF and MTL. If you're unsure about the optimal ratio, start low. After draining, put 3 qts D4 ATF in, and then pour as much of the 1 qt MTL 75W80 GL-4 as you can before fluid starts running out of the fill hole (~0.85 qt). Then drive in the cold and take notes.
The next time you change the oil, you can increase the ratio of MTL if you want. I've found that 1 in 5 people don't like the cold weather performance and should reduce the MTL to 0.5 qt; the other 4 aren't bothered by it. 5 out of 5 love the shifting performance--I've never heard someone say they didn't notice a significant improvement.