Lowering an existing aircraft's performance

Hello,there was this aftermarket developer who used to make 777's. Only they were the LR model and as we know most have the 200ER model.I have already managed to merge it's panel with Overland's model. I am now in the process of trying to correct the engine and weight specs to the ER model instead of the LR.Editing the weight isn't much of an issue, but I ran into problems with the engines.My thinking was that if I copy the [GeneralEngineData], [TurbineEngineData] and [jet_engine] values from the Overland .cfg file into the other .cfg file I would get the correct engine specs (at least according to Overland) for the 77-200ER.But when I test flew with the edited aircraft.cfg, the aircraft struggles to reach 300 knots at FL270, of course this shouldn't be the case.My question is, is it harder than I thought to change the LR cfg into a ER cfg. Do I need to edit the .air file as well?Thanks for any help or pointers,Xander