there are quite a few computer simulations of air flow in intake manifolds and the effect changing direction has on airflow, Very interesting reading.
My observation or question is: Is the tube feeding each throttle body oversized x3? The intake air volume is divided in half and the tube feeding three cylinders is the same diameter as the indivdual throttle bodies.
A colume of air in motion has mass so turning it 180 degress to feed the front 3 cylinders will affect air flow. So at some point in the rpm range it could be the limiting factor to producing power
The comment about a wall 15mm ( less than 5/8 of an inch) from the air intake dosn't have an effect is contrary to what has been tried in motor sports for a long time.
Of cource you can't build the mathimatically ideal shaped plenum for the rpm range you want but the internet has a wealth of information about trumphet length and plenum volume. Good luck and keep on welding.