It's the plastic arms. Replace them with aluminum ones and your surging problem will be fixed. The plastic can't handle the torque of the motors especially if you're loaded down/heavy.
You'll want to get them for the 550 once you see them in action on the 450. I'm doing the opposite, got them for my 550 and will be getting them for my 450. You'll be able to upgrade the motors and props after that and realize this things full potential.
My f550 w/ naza has the same problem. I repositioned the lipo and it seems to have solved my problem. Would off center cg cause this? I'm thinking that one motor is compensating for the imbalance causing it to yaw. Not sure though as I'm still new to these.
My S800 is connected to the Rx via sbus. The Zenmuse is connected to the Rx via 8 servo cables. Is there any reason I cannot connect the Zen via sbus and get rid of 7 servo cables?
I just replaced the bearings in one of my motors and they are 4x11x4mm. They come out very easy with a press, or using a table vise, sockets, screwdriver and rubber mallet.