I would wager that the quarter grounded are the same quarter which have recently been given the mk 106 Adour engine. This gives an extra 6 % dry thrust each side.
A Jag mate told me a few weeks ago that the extra weight of the exhaust negates the extra thrust produced. (Made of/with titanium I think he said.)
Its true that it is underpowered. They struggle to get airborne from Incirlik with a full load. (At Colt too I believe, on a hot day)