Jan,
Its seems the most logical thing to try under the circumstances. You have pretty much eliminated the engine as the culprit. Too close and you may see an EGT rise/thrust loss.
I would move the engine 10% of the current gap at a time. It may very well be something else since I do not know the rest of your system.
But keep in mind the basic effects as I explained them. You may hit the sweet spot first time. If the inlet ducting is too small in cross section area it may remain an issue.
At the other extreme if the engine is so far forward so as to limit bypass air it may also improve.
Hope it helps.
Andre