In the past, most PV modules notoriously didn’t match the manufacturer’s nominal specifications. The real behavior of modules, with respect to the specifications, was one of the greater uncertainties in the PV system performance evaluation. Today, most manufacturers practice positive sorting, rounding down modules rating to the nearest multiple of 5. For example, a module measuring 313 Wp is rated 310 Wp and a 309 Wp module is rated 305 Wp.
With positive sorting, manufacturer guarantee and increasing use of independent certifications, there is much greater certainty on module quality nowadays. The gain is however small, in average for a 315 Wp panel, it is only 2.5/315 = 0.8%