They take your most accurate gear stat (e.g. slash, stab, crush, range or mage) to calculate accuracy, max hits are calculated by your ranged level. That means your max hits and therefore your exp per hour will drop because sara brews lower your range level. If you want to do this, use super restores instead of prayer potions, they give an extra prayer point, cost only a little extra and counter the stat-lowering effect of brews.