No light twin certified under Part 23 has much performance on one engine. Unless, and until, configured for min. drag and flown correctly you can expect to go downhill. The Navajo is no worse than most others.
You can, however, improve its performance with the Panther upgrade. It replaces the stock 310 hp or 325 hp engines with a Chieftain's 350 hp versions. You can also add a VG kit which will allow a greater take-off weight (and still meet certification performance requirements) or use the slight additional performance at orignal weights as a bonus.
I would have expected a correctly flown PA31 at MTOW to fly OK with only a blown turbo. It would need to be flown correctly though. Also, are you sure it wasn't overloaded? 6 bums + lots of fuel will do that.
I had an inflight failure a couple of months ago in a Panther Navajo I manage when an injector line broke. I wasn't far off MTOW (4 POB, bags, fuel fuel - 1 hour). At 9000' I needed full power and precise technique with heat becoming an issue, 8000' climb power but heat management was a bit of a pain, and at 6000' 75% power with temps in normal range at Vyse+10-20
I wanted to burn fuel before configuring for landing to improve performance with gear & flap out so chose to airport hop back to home base with maintenance. There were some two dozen airports we flew past in the hour+ it took to return so we were never more than a few minutes from a landing point if necessary.
It really wasn't a drama at all.