If you were to get two bows of the same model, one, a 70 pound bow backed down to 60 pounds and the other a 60 pound bow set properly at 60 pounds, using the same arrow the 60 pound bow will shoot a little bit faster than the 70 pound bow will that was backed down.
I did that very thing when I bought my bow last year. 70lb Drenalin cranked down to 62lbs with a 400gr arrow shot 271, the 60lb maxed out 62lbs and shot 273. Both bows shot the same, had the same feel, and were no more or less louder that I could tell.The reason I went withthe 70lb limbs was from past experience,the 60lb bow will only max out at 57-58lbs a few years down the road from limb wear, new string/cables or not. I'll always be able to keep this one at 62lbs.