I used the 3121 all around on my Madness with an OS46FX. The final weight with Duralite was 2.2Kgs. I had to add one and half bar of lead in the back for it to balance correctly (I had the rudder servo back as well).
I crashed it when my thumb hit the Rx switch on a vertical take-off.

Seems like the Flip will be at least 2.5Kgs by just weighting the separate parts. I switched to Hitec 525 because I didn't want to re-work the pre-cut holes. I think you guys are going too deep with the servo, I would take fast over powerful if it was me.

I don't know if the moments are the same on the Flip than the Madness but my concern with a 4 stroke will be that more weight will need to be added on the back for nice and easy hover. I don't think that the 5&1/4Lbs is realistic but I might be wrong.

From a first view, the kit is well built but the hardware is extremely heavy (full metal hinges, never seen that before!).
I am building mine completely stock and see where I will end-up.
I was able to hover the Madness at about 1/2 throttle with the 46FX using the fun-fly prop (12.5 x 3.something??), don't know how much the added 250grs will damage the 3D performance yet.
I just finished the wing, got it on the fuse and I have to do all the rest.

BocaBen