A consultant hired by Miami’s firefighter and police pension fund dropped a bombshell this week, estimating that Miami taxpayers owe more than $200 million in back-benefits to retirees following an invalidation of controversial recession-era austerity measures. The NYHart Company’s report — issued just days before a consequential city election — puts a partial price tag to changes set in motion in March by a Florida Supreme Court ruling that struck down pension fund cuts made by city commissioners in 2010.