Personally, I use your same equation, 45# recurve performs like a 50# reflex/deflex longbow, everything else being equal (which of course it never is). But after shooting many different bows and cronoing many of them, I don't think that's far off the mark. Certainly I don't believe there is MORE than a 5# difference, probably less: maybe a 46 or 47# recurve is equal to a 50# longbow. Who really knows, because there are some rd longbows that shoot faster than some recurves, too.
Speed, of course, is only one factor to consider when switching from a rd longbow to a recurve. While you gain some speed in a recurve (after all, those recurves on the ends of the limbs must be doing something!), you also gain a greater propensity for limb twist, so there is a tradeoff. As there is within recurves and longbows themselves. For example, BW could design the PCH to be faster than it is by reducing the amount of deflex. But the reason people love BW recurves is that they are fast enough, but super reliable and consistent, largely due to the deflex built into the bow.