The FSX engine IS a great piece of software....but you have to remember it's 10 years old now: It was designed at a time when the expectation for hardware was higher clockspeeds and single CPUs and before internet delivery of software took off (at least for multiple GB of data). Furthermore Outerra doesn't (yet) handle the vast array of subsystems that FSX does (i.e. AI flightplans, AI shiplans, AI road and boat traffic, Air Traffic Control, landclasses, integration of higher quality DTM, aerial photo or landclass data, seasons and weather etc etc.
Apples and oranges really but the guts of FSX + fractal refinement and lighting of Outerra would be awesome.
I'm not the most tech savvy person out there, but I'm pretty sure that anything AI shouldn't have an impact on current performance as the CPU should handle that (and it's pretty free in terms of used computation power), having more models being rendered doesn't seem to tax the performance a lot either (at least based on the tests I've seen here made by other forum memebers). Same with the rest, performance test so far clearly show that Outerra is highly efficient at what it does.
The problem on a straight comparison is that Outerra is a WIP engine and FSX is a finished simulator with years of development on top (both from MS and Dovetail). The later will inevitably become redundant eventually because that's how technology works.
If starting -sort of- from scratch to properly handle new technology (be it graphical features or whatever) wasn't necessary, Valve would still be using the GoldSrc, Bohemia wouldn't be trying to come up with a new engine to handle larger landmasses+longer range rendering and Fallout 4 wouldn't perform so horribly bad (because that's an old engine with a lot of glitter on top).
Just my opinion from a player's perspective, I could very well be wrong, but that's what I can see from here.