Well since it wasn't specified, my vote for 12-15psi max was billed for the stock 7.3. I would not run that on a stock 6.9.
My pickup is in my sig, bought N/A 12 years ago. I run the heck out of it with 1/3+ of its life pulling a 4.5k trailer. Sometimes cabover camper+trailer. I've put over 100k on it in that time. Now(last two years) with the turbo, about 3/4 of the miles have a 4.5k trailer or the same trailer empty at 2kish. Shes a dedicated work rig now. Way easier to write off the mileage this way!
Got my 94-era Factory Ford turbo kit for $800ish shipped on Ebay. Long story short it was SUPER clapped out and I got a new center section from R&D for $600+10% tax because I live in WA and our sales tax sucks. So nearly $1300 for a stock Ford turbo. Not a good deal.
Before original install i opened up the exhaust with the 3" elbow mod, and 3" homemade pie-cut down pipe. It was a pain but a fun welding experiment/practice.
Could only get 5psi before new center section, then up to 9ish with new.
Maxed out the pump and unplugged the waste gate and got 12+ depending on the day and variables I still don't understand.
Some days i would get up to 15psi but it didnt feel any stronger than 12 or 13. Too much not air.
I bolted the wastegate shut and got much better spool, with 15psi commonly. It still wasnt more powerful with 15 than 12-13 but those are the numbers.
Enter the charge air cooler! Or CAC. (Most commonly and quite incorrectly know as an "intercooler").
After this my boost levels went down a few psi, but power went UP. Egts didnt change a lot yet, which made me wonder...
Thanks to
@Thewespaul and others agreeing with him, I cleaned up my crossover pipe and up-pipe seals and added copper RTV to those mating surfaces. The difference is HUGE. Nearly as big as a fully clapped out turbo and a new center section.
I also un-bolted the waste gate and just run the turbo with the gate tube un-hooked. So it blows open at around 12-13 psi depending on ambient temp. But the engine pulls harder than ever and spools harder than ever, too. I can get 3 psi at 1300rpm which is just awesome compared to anything before the exhaust sealing and CAC.
So that's my story. I hope it helps others! I un-bolted the WG on Wes's advice that it's a good safety measure to keep from over-spooling the turbo. We have yet to get to the bottom of that but I'll leave it as-is for now. This pickup runs great!!
I may have to edit this later as it's late and I kinda went down the rabbit hole, lol...