Remember that higher gain means more directionality. Higher gain omnis produce a flatter doughnut shape so will radiate a lot less energy in the vertical plane. If you have a one-floor appartment that may be a good idea, but given you say the Omnia is in the corner of the house, any onmi will be radiating a significant portion of its energy into the wall away from the house. Replacing the low-gain omnis with low-gain MiMo patch antenna with say 120 degree beam width would allow you to transmit the vast majority of the energy into the rest of the house without losing wide angle coverage there.