Im not sure about Gernot setup on his Katana, but in this Sukhoy he really flew with 6S packs, that way he was under 5kg limit, not remember what capacity he use but it was enough only for little more than 4 minutes, perfect pack for Artistic aerobatic 4 min routine.
Do you remember what servos did he used ? I have the same model just v2 , and right now I don't know what servos to use because the ones that I bought for it have a ugly overshoot. What servos did u use on you're EF Extra ? Thanks!