I have two MT Extra300s. The one with an OS.25FX still flys great. Even threw it in the trunk and took it along on my vacation to Minn/Wis last June. Should have seen it buzzing Lake Superior.
The other Extra is another story. I put a .40 in it and had around 10 flights with it. It balanced out using the baterry and RX toward the rear. No extra weight. Very fast !!! Bad part is that even after extensive glassing and added support , the .40's vibration habits took their toll. Had flown a couple of hours with this bird one Saturday,,, no problems. Until the vibes caused the RX/Batt tray to drop down onto the single aileron servo mounted on the wing ,, keeping it from moving. And when I did get it to make the turn it stayed there all the way to the dirt.
Get a 40 sized profile and have some fun with your FX.