When doing 45's you've got to have the port width in the bend as precise as possible, so user error plays a big role in any loss or gain. There's just too many variables involved to prove that the 45 is to blame. I use them in every box I build, if only for baving extra bracing. I use body filler to fill/smooth where the 45's meet the port wall, but I can't say whether it adds any gain/loss.
As far as the resin goes, I use it to seal the inside/outside of the box of it's going in a boat/UTV/jeep to help with resistance to moisture.
Sent from my Motorola 2950 Special Edition 3w