You won’t find an improvement of 30-50% by streamlining in front of the prop. I’ve had to ride like this when i didn’t have the correct spacer for the prop with me and it works, off course not optimally and it’s worth fixing it.
I wouldn’t say that the board size makes a difference that is important just to foil, not the mast position either. A big board is nice for paddling, nice to learn on since it’s easier to drive it on the water, nice in rough waves and nice if you ride in the winter. I’ve started at 180cm, now after two cuts i’m at 150 on my longer board.
A mast more to the front makes the balance point for weight and the float of the board better centralized, and it gets the prop a bit further in from your feet, not that these are critical but there are some benefits to having the mast a bit to the front when you have a board that can fit it.
(I have tried from 30cm to zero offset from the rear, on my 115cm board it’s all the way to the rear, on my 150cm board approx 20cm from rear)