I think a 600ma battery under the tank is ok. I used a 1000ma and this made it a little nose heavy. With a 46fx and standard servos mounting the battery directly behind the fuel tank is better.

I think the under fuel tank mount is when you use a 40LA or 32FX motor. The only problem I have ever had with the plane is snapping aileron hinges. This is rare and I'm the only one at the field that has had problems. But I like doing a snap spin into a vertial dive with full aileron and full rudder, the spin rate is great.