>> I an many of the highest levels can 2-3 shot the common NPCs. The Flamehawk takes me 8-10. Thats 3 to 4 times as much energy. <<

Based on some data, the average base XP for firing a shot at NPCs is 2.1 (kill bonus excluded)
Based on 10 shots (impressive), your effective XP ratio for the FlameHawk is 2.7 (3.375 with XP buffs, note around 3.64 is enough to sustain ranking continuously with certain builds).

The FlameHawk (30xp) is the only high rank unmatched common, and this is probably why the high frequency (perhaps only 1/3 spawned are matched?); I'll assume it's max rank is either 1) no max or 2) like Sha'din Hypergrid, rank 1200, which we will consider unobtainable for the moment.

Other commons: Sha'din Charger (30xp) and I'm assuming Dark Runner (35xp) can still spawn regardless of rank; matched.
Fairly Common uncommons: Trapper and Trafficker I guess; matched.
Rare+: Low enough spawn rate not to significantly affect XP outcome, I think.

In the example given, 8 of 13 were FlameHawks.

I am assuming 4 shot of uncommon matched, and 2 on chargers because you're insane like that

Flamehawk x8 (30 xp, 45 energy average/9 shots)
Trafficker (50 xp, 20 energy)
Trapper x2 (50 xp, 20 energy)
Charger x2 (30 xp, 10 energy)

240+50+100+60 = 450 for 440 energy. Let's estimate 1.0 kill bonus XP ratio. Therefore, 3.1 overall xp/energy ratio (3.875 with buffs, enough to power rank). Although, it's debatable that someone with an engineer build and a race that gives energy/xp increases (rather than Drannik, which I'm assuming is how you're killing them with 8-10 shots) would be able to kill these as efficiently.

If we had more data, say 50-100 NPC runs, we could make a better estimate of the spawn rates, but it may still be possible to continuously rank despite the flamehawks (they are unmatched which helps since you can get stronger to kill them more effectively over time).

As a side note, you can now refresh 6 NPC slots every 8 hours if you have finished certain missions. While this wouldn't help much in a continuous NPC hunt or while trying to use a full energy bar within XP buff timers, it does skew the results slightly since you can eliminate 6 flamehawks from a run.

Also, at high rank you might have a lot of artifacts; a Drannik Sab would make it take about half the normal amount of shots, but again that's a minor adjustment and you only have so many.

-- I still would like to see more content, either matched or rank 400 though, develop over time.

I think what makes power ranking possible is either a complete lack of defense because you don't have decks or a long long history of rescued prisoners and ship-bots. That is before the flamehawks come into play, which basically screw everybody over that didn't get to 350 before they were implemented.

What about rank 350 ? Well I'm at 360 and there are only two missions left with a high enough XP ratio to rank multiple times at once. So, the power ranking from missions stops before rank 400. Then we're left with oddly enough Stellar Cartography at 2.33 ratio as a repeatable mission that's better than most of the misc. remaining missions (at least until Dan adds something new with a nice ratio). That'll get you 80% at least to the next rank with XP buffs, it's between that or dealing with FlameHawks. With patience I guess you can wait for them to expire but they are still more frequent than anything else I think.

Power ranking without energy refills. before flamehawks it was possible to rank nearly infinitely as long as you had neural interfaces and a way to repair. you get enough RSL implants just by killing that many NPCs every level. You just need enough engineers. I'm sure a lot of people did it before it became impossible. Basically only the top .1% of the game benefits from every nerf that has ever happened.

I'd feel better if it dropped a crimson amplifier all the time, it'd be the new obelisk and would at least feel kind of rewarding. I wish I had more data on spawn rates at certain ranks to make an analysis, but I figure they appear at least 50% at higher ranks.

There is one very interesting idea, based on how much mission NPC seem to interfere with normal spawning; leave some missions on that spawn stuff that you can tolerate (almost anything is better than flamehawk) and farm as normal, it should dramatically increase the yield.