I suggest a short test to see what the problem is. Here's why:
I have a 2002 Saab Viggen (T7 ECU) which also backs off the power during very hot weather. I live in Southern California where I'm stuck with CA91 octane. (Realistically it's more like 89 octane everywhere else.) I get very similar boost retardation to you. I have found a temporary solution for summer months that basically takes my octane rating to your level. That has eliminated any problems and gives me lots and lots of power. I suspect you have a different problem, but propose a way to test it out ...
Test: Buy some 100 octane fuel, 5 gallons of Toluene, or Torco race fuel in a can. If your boost goes up immediately after fuelling up with some high octane fuel you will know for sure that knocking is the problem.
If your boost does not change: Then your map from the tuner was conservative. This might be because the the tuners, living in Sweden mostly, do not have the right conditions to test your ECU in hot Luisianna weather.
If your boost does change: Then it was indeed a knocking problem. I would guess that it was either excessive heat from your A/C condensor (which sits in front of the radiator on your car) throwing too much heat onto the intercooler. Or ... that you need to move your nozzle farther away from the intake on the engine. Or ... that the combination of a VERY rich mixture and WI has caused your combustion gasses to burn so slowly that they're overheating the exhaust valves and causing hot spots which only make detonation worse. Hopefully the latter is not the case.
Just some thoughts on the matter. Try the test first. If octane makes no difference it's most likely NOT a problem.
Toluene can be bougt in 5 gallon drums from Sherwin Williams and Torco race fuel additive (which I use and is just about as good as 100 octane if you start with 93) can be bought here:
http://www.need4speedpower.com./torco.html
Kevin Yankton also had some limited Success with WI on his 9000 Aero. With just the same mods as you plus an uprated intercooler and intake manifold he was running 25+ PSI on crappy CA 91 octane. So the WI just work, just a matter of figuring out why it isn't for you.
Here's his site:
http://www.kyankton.net/9000index.htm
Adrian~