Well if the solenoid has been replaced and all vacuum line are good, my only guess is that the vacuum is
Shot. A google search will show home made mechanical wastegate with a spring. Very easy to make
First off your gonna need a boost gauge. Second you should do a google search for 6.5 mechanical waste gate or go over to diesel place .com. That site is dedicated to gm diesels
Yep it's that coil.
Changing up the exhaust like that really can't affect how it runs all that much. I did custom exhaust on my truck along with deleting rear o2 sensors and she purrs. No tuning either