The upper weight of the Katana is 7.5 lbs and the gas engine will probably take it slightly more than that so its not surprising that 3D is not possible. On the other hand, the Seagull Funfly 3D has an upper published weight of 6.7 lbs though having 15 more sq inches. That 3/4 lb difference might be significant if the numbers were good.

I've finally got all the needed hardware to assemble mine so will have an all up weight in a week or so.

3D possible or not, it should come out fairly lightly loaded and nimble.

Edit Correction: The upper published weight of the Seagull Funfly 3D is 5.7 lbs rather than 6.7 so there is 1.75 lbs difference between it and the Katana.