Another benefit you gain is much faster accumulation of engine reliability data. With the first test launch you get a statistical dataset of 81 burns. With the second test launch the dataset is 162 burns. That's a better data set than we had on the SSME's 50 launches into the Shuttle program. Very early in such a system's life, you could probably stop doing engine tear downs and inspections between launches because you'd have a good handle on the expected failure rates, and the large numbers of engines means you expect small numbers of engine failures as a routine part of operations.
This is not entirely true. You would get a data set of 81 initial
burns. Not the same data set as firing the same engine 81 times. The former would be of limited use for reliability testing of reusables.
The Falcon 9 actually has engine out capability only after the first 30 seconds of launch. Before that the T/W prevents 8 engines from providing enough lift.
It's probably moot anyway. If NASA asked SpaceX fora rocket with that large a payload They would clean sheet it with a larger core diameter and bigger main engine just to avoid the functional nightmare of a frankesteinian 81 engine, 9 core system.