Tougher than nine hells though?
Now THAT'S thinking high-level play!!!
And, no, probably not on most days.
Four Revised Pit Fiends (in your estimation) are approximately equal to a party of four 16th-level PCs.
According to the way the CR/EL system works. Yep. Granted, the Pit Fiends are tougher in many aspects, particulalry their well-roundedness. However, our party is better in each respective area (Fighter, Arcane Spellcaster, Rogue, and Cleric) and furthermore will probably have even more synergy because of the way they work together better.
Having said that, I would have no problem conceding certain key strengths to the Pit Fiends in comparison (Meteor Swarm, High SR, Special Qualities, and Insanely High Attributes).
Yet you believe a party of 16th-level characters would be a match for them?
A match? 16th Level Party against an EL 20 encounter. The party could win, but they'd need luck and decent prep to do it. More often than not, however, I think the mortality rate would exceed 50% for that kind of encounter.
There are only three monsters (other than the Gem Dragons which I am not going to study for the sake of brevity) within the entire Monster Manual 2 which come close to rivalling the challenge of a Revised Pit Fiend:
Hmmm....
Tempest, page 193, CR16
324 hp
24 AC (Ok, that blows, no pun intended...really)
4 attacks doing 2d8+13 (21x4 = 84 hp avg capacity in melee)
Only good save: Fort
SQ: Elemental, DR 15/+2,
Str 29, Con 28, the rest crap
Whirlwind Ability...which is sick
So, in comparison...the Pit Fiend is far more well rounded, however the damage capabilties are very similar, and the tempest has a wicked-tough damage shield that deals damage AND negates without a save Ref DC31! This effectively takes out most front line melee PC's!
Tetramorph on the next page is in the same ballpark. Awful AC, 300+ hp, SR 32 (sounds familiar), and a wicked tough attack effect (Entropic Touch).
Most of the MM2 monsters have one really sick ability, and a variety of defenses that would be required at the appropriate CR level to "compete".
Don't get me wrong. I'm not saying EVERY CR16 should look like the Pit Fiend. No, I want the Pit Fiend to be the high end of CR16 (much like the Dragons of appropriate age category). However, all CR16 creatures need to look much closer to this Pit Fiend, than to what was published originally in MM.