........ The car still runs fine and builds boost up until I hit 15 lbs then it does the same thing. There are no error codes and Scanmaster numbers are fine. Could I have blown the stock turbo? Hopefully it's something simple.
When using e-85 in our turbo cars you need to realize there are many factors to consider and your symptoms are typical of what we have seen in many conversions.
The first thing to consider is the alcohol is MUCH harder to ignite than gasoline. This mean your ignition system must operate at optimum so you have enough spark especially at higher compression when the boost level in increased.
I have seen at least 1/2 dozen converted turbo cars exhibit the exact situation you have encountered including my e-85 500+ HP street car. After many "changes" of various parts including 3 different new coil pack/modules, it was finally cured with the installation of the TR6 ignition.
Like the 6 examples I mentioned above, all those were cured by the TR6. A few other street conversions that are set at a lower boost have no issue.
Another point of reference is my race car the used 116 octane for years and only had to change the program for e-85 to run the same or more HP with no other changes. Certainly the MSD distributor ignition has an awesome spark, so NO issue there!
One issue I did find in the race car and other e-85 conversions is contamination in the fuel, mainly small rubber particles, which the alcohol has etched from the rubber lines. My street car has a new fuel tank and ALL new lines to prevent this issue, and all the race car old braided SS fuel line were updated.
In your case with a stock turbo and intercooler, your inlet air temps are sky-high even at 15 psi which will give you detonation, and even worse you are on the edge of pre-ignition where you do NOT want to be.