If this is the default tune and no fuel controller I’d see what other tricks are available. Last thing you want to do is lean out under WOTIt will need to learn. The way it works is that the ECU has a programmed fuelling map that, assuming everything is as it was when it came out of the factory, will keep the fuelling correct under all load and revs conditions. The short term fuel trims alter to keep the mixture correct under all conditions but if they are always adjusting one way, the long term trims then adjust to get the short term trims back to the mid point. Resetting teh trims puts everything back to default so it will need to learn. It needs a mixture of conditions, cruising at small throttle opening, light acceleration, heavy acceleration, cruising at high revs and an uphill load to fully learn. On initial startup it will also run a default cold start map, and the only adjustment you have there is getting the air valve steps correct when warm which will put it in the right operating range for a cold start.
I’d ask XJREngineer on the jag forums, he knows GEMS better than most of us.