""I was able to find out that the balance point is 5.75""

umlaut,,

(so we are talking about the 73 inch span plane) at 5.75 I don't know why the balance is so far back, I say that because that plane is so long !!, where did you get that info ??
I balanced mine at 4" & 1/4 because "as I was told" many and most planes of this size are balanced about there, I only flew mine once but to me the Balance seemed OK at 4 & 1/4,,

mine weighed around 15 & 1/2 pounds and had a G38 in it and I needed to put my battery way up front to balance it at 4 & 1/4, but then again mine could have been built Tail Heavy ??????

as for the 108, I myself would think you would need more power, specially if the weight gets up around 14 pounds, but there is a guy on RCU that has one with a 120 4S in his and he is happy,,, so,,

the first thing you need to do is add Shear Webbing, I had a problem with wing flex, it flexed on the ground so I know it flexed in the air !!

in the air,,
in the air if flew rather well, meaning it was not that sensitive at all, and the faster it flew the more stable it became, but if ether wing dropped a bit the plane would just want to go in that direction, meaning if the Right Wing dipped a bit the plane would want to veer down to the right, I contributed this problem to the Wing Flex, yet this problem was not so bad that I couldn't stay on top of it,, other than that the plane flew fine,,
I did get to ask Walt Moucha on the phone about this problem and his answer was "well it is an old Racing Design you know, and that is what the sticks are for",,

Engine Thrust Angle,,
now this is where I had a problem, my plane didn't have any Thrust Angle in the Engine, and as I found when I was coming down to land and the plane was off the Throttle and just some 5ft off the ground and slowing the plane would veer Left, after researching this on the net I found this is a long known problem with planes without Engine Thrust,,
I don't know a lot about Engine Thrust Lines but the planes I have owned from 60 inch span to 82 inch span had 2 to 2 & 1/2 degrees of Thrust,,

anyway because mine veered off to the left when landing I hit the high grass, I now have to build a new wing and a new Engine Wall errrrrrr

I hope this helps !!
Jim