The basic hull of the Cutter seemed to be already fit for every purpose that they had for it, not like some of the earlier designs, which couldn't hold up at higher speeds or in certain new situations.
This one was nearly perfect already, and their Warp Speed travel was well above average, though the ship's Warp Drive hadn't been optimized for extreme speed. It had more than enough power for Portals, weapons, shields and everything else that they had asked of it so far. By that standard, it was hard to complain that the Engineers didn't go further in trying to make it extremely fast.
It was designed as a scientific research cutter, not a military interceptor. Those two might be the same thing with Max and Nico on board, but that was beside the point.