It is possible that the older Miranda-style core that this thing might have had was too underpowered to drive the newer engines. It then begs the question of why did the Starfleet Corps of Engineers go to all the trouble of bolting on scaled down Galaxy engines (they were probably left-overs from an unfinished Nebula prototype
stuck in a drydock somewhere) without beefing up the core to support the load?
This may actually be touching on the reason the model was never used. The dialog was already written when Data described her as too slow. Then, when the producers saw the model with 24th century Galaxy components, they maybe felt like it didn't look like it should have been "too slow". It was likely just a throw-away line that didn't mean anything in the grand scheme of things, but the bridge footage was probably already filmed and put together and they just opted to save some money by leaving out the extra visual effects of a new ship that didn't quite match implied dialog (and they probably thought it was too ugly to show on screen, too). It is, quite possibly, one of the oddest kitbashes I've seen in Trek, rivaled only by a small handful of some of the more bizarre DS9 frankenfleet members.