i don't think you should be disappointed in the bows at all,, the sinew bows are performing well for the amount of reflex they are holding,, and I am sure smooth to shoot,,
I don't know about the second bow shooting the heavy arrow faster,, but that is a very respectable cast for a heavy arrow from the 53# bow,, the bows are shooting great, you can fine tune for better cast on your next ones for sure,, but both are very good for hunting bows,,
yes I guess if the heavy arrow is a better spine for the bow it might have something to do with shooting faster,,if it is coming off the bow smoother,, ??