Don't put too much weight on the difference between Marty's filtered and unfiltered performance.
The filtered system is crippled, taking its air from inside the lower cowl volume. That volume has already suffered a large pressure drop across the engine cooling fins. It is being fed with little more than freestream pressure, unlike the standard snorkel filter, which is at freestream plus 70 to 80% of the available dynamic pressure (q).
In addition, the filter is in the cooling air outflow from #2, which is quite hot, so air density is low....effectively the same as a big jump in density altitude, and pro-detonation too.