I have an older 65lb non-fastflight recurve bow that, with an arrow of about 550 grains, is slightly slower than a newer fastflight bow of 56 pounds. I think several other things contribute to the higher speed in addition to the ff string, including better materials and the designs used today, I draw the lighter bow about 1/2 inch farther, etc.
I would recommend you shoot them side by side. You may find that the 46 lb bow will provide the same speed to a heavy arrow.