That Harken piece is nice. However, if there is no significant change in direction, a simple fairlead won't add any friction. I have a real block where the furling line comes off the drum and then just fairleads along the stanchions.
You can easily detect how much friction the fairleads or blocks add by just furling the sail by standing near the bow and pulling the line where it comes off the drum.
Fairleads will definitely cause more friction that a good leadblock unless you have a straight line. I have confirmed this more times than I can remember.
You are correct that you can detect how much friction the fairleads or blocks add by furling the sail standing on the foredeck by you have to try it in all conditions.
The Harken blocks look pretty good to me. Side decks are narrow enough to begin with.