So I'm 3/4 of a tank through and performance has improved significantly. I would say it improved quite early on in the tank and has been getting better and better.
Here are some details that someone might find interesting:
I used an entire bottle of Gumout All In One that can treat 35 gallons in an 18 gallon tank.
It's been in for 3 weeks now.
It's done a mixture of freeway journeys and short trips.
I have several WOT's and on a 15 minute stretch on the freeway I disengaged OD and drove at about 80mph which was over 4k rpm.
Both performance and fuel economy have improved. The engine clearly spins more freely. I can't unfortunately hold gear apart from disengaging OD and I haven't been able to drive on an empty enough freeway or other road to do several back to back WOT's to the redline.
I plan to do another dose prior to the next oil change. I'll buy the same All In One but this time use the normal dosage and use the other half in my other car which previously got Techron Concentrate Plus.
I've also located a cheap Chevron gas station and will try to use that in future.
One of my observations though is that there is probably no need to do WOT's, especially since none of the additive manufacturer's mention it. I think that normal driving patterns are fine.
Another observation, on a car I previously owned from new, which had direct and port injection, I had never used additives and only sometimes used top tier. Prior to returning it, with 40k on it, I decided to have some fun with it and drove it near the redline on the freeway in 2nd gear a few times. The next day, it immediately felt better.
By doing WOT's or an Italian tune up with fuel additive in the tank, my sense is that some of the cleaning effect is going to come from the rpm regardless of the additive.
In these cases, it's hard to actually say whether the fuel additive did anything.
Lastly, I like the idea of using the All in One as it has a friction modifier. It is also cheaper than the regular Gumout ie 6 oz of regular treats 21 gallon, while 10 oz of All In One treats 35 gallons. Based on the bottle quantity to treatment ratio, I would say that it contains the same % of PEA. So for $8 you get two treatments plus a friction modifier vs for $5 you get one treatment.