With that setup, you should be using "spine group 5" arrows, according to Gold Tip's sizing chart.
If you drop it to 56 lbs. that could put you into "group 4". Your kind of on the borderline. The only way then to really tell it's effect is to paper tune. Find out which spine group arrow you have.
But then, all of this depends on how accurate and consistant you shoot your bow. We may be splitting hairs here and it's not like your competing. Have you target shot your bow after making the weight change? I would try shooting for bull at various distance and if you do good and are pretty confident you'll hit what your aiming at, then it's effect is probably making little difference to your shooting.
Did changing the weight make a difference? I'd say it most likely did. But it may be a negligible amount.
Tiller is the distance from the limb pocket, where the limb intersects the riser, out to the string at a 90 degree (perpendicular) angle. So yes you are correct about that and also how it's measured. I believe about a quarter inch difference is exceptable.