A healthy GTE should get better mileage than the GE... they did from the factory.
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Your horrible mileage is due to the engine problems, not the turbo.
Running a lower octane will generally increase your mileage as well.
I'll try to help with individual questions, but I haven't the time to post a writeup or to watch the thred like a hawk. IMO, 95% is obvious.
I recommend keeping the oil cooler setup from the GTE.
Are these from the same revision? Pre89 to pre89 or 89+ to 89+??