As Dan alluded to, external stores, such as this fuel tank, reduce the speed at which the wing will flutter since they reduce the wing's torsion frequency, and the wing's torsion mode will couple with the wing's bending mode at a lower speed. (Classical flutter involves the coupling of two or more modes of vibration. For example, the wing bending mode and the wing torsion mode, or the aileron rotation mode and the wing bending mode.) It appears that the fore-aft center of gravity is slightly ahead of the wing's elastic axis, and that is beneficial and will offset some of the adverse effect mentioned above, but the overall result will be a lowering of the wing's flutter speed. I would strongly encourage an assessment/analysis, and possibly a ground vibration test, by a knowledgeable flutter engineer, and only then open up the flight envelope carefully using accepted flight flutter test techniques.