I have a 61 on mine... added 1 ounce of lead to the tail...
flies nicely...
IIRC the balance is per spec... but I think the spec in the directions was wrong...did you get the corrected number from the website?
I'm not at home so I don't remember the number, but it comes out to be just about the middle of the aileron servos
13 x 4w prop is excellent for it..

flat spins are great... what I do is give the aileron and rudder to start it, but once it is going, bring the aileron back closer to center to keep it flat....

can even do ascending flat spins...inverted or upright!

Here is a direct link to the manual
http://www.accelhobby.com/manual/katana_GS_40.pdf
Just found it... the point is 130mm from the LE