Upgrading the weapons to particle cannon but with the same missile loadout might be all right. It still should have less shields than the Hellcat and less armor than the Thunderbolt (or in WC2 era, shields about the same thickness as the Sabre). Speed and maneuvering-wise it would be about equal to the Thunderbolt (a smidgen less cruise speed and a smidgen more afterburner speed). Roughly this would make it moderately inferior to the Hellcat all-around except for having slightly better armor, which would explain why Scimitars were mostly withdrawn in favor of Hellcats where possible. As far as viability goes, any of the pre-WCIV enemies that have turrets (notably the Gothri, Vaktoth, Paktahn, Sorthak, and the WC3 corvette) would be trouble but against anything else it can come close enough to matching on protection and firepower if not speed.