To be sure, the E-D was a big bad mother, and the Klingons in "The Emissary" fired from their maximum range. The Voyager is among the smallest known Starfleet ship types, and the Klingons fired at point-blank range.
We haven't seen how Klingon weapons of the TOS era (to which this ship was native) fared against their Starfleet counterparts, because there was no straight-out firefight where the Klingons wouldn't have been pulling their punches. Okay, in the remastered version there was - the originally invisible ship of "Errand of Mercy" was transformed into a D-7 and her failure to destroy the hero ship might thus be counted. But ignoring TOS-R, we might argue that a Klingon battle cruiser could destroy her Federation counterpart with a single volley; the opposite appeared to be true as well. The performance of the Voyager would thus mean 100% improvement in shield protection... And, as we saw, it was tactically decisive in allowing the Federation ship to triumph over her similarly sized counterpart.
Timo Saloniemi