This really has me concerned about the ability of PCs to function.
If that is still a CR 16 creature, it's SR will mean that a 16th level wizard with greater spell penetration will need to roll a 16 in order to effect it with any spells. Without Greater Spell Penetration, there's no point in even trying. And the wizard had better have his spells prepared as Sonic damage and have Greater Spell Focus in the school or they probably won't do anything anyway. Heck, even with all that, the odds of both getting past the pit fiend's SR and getting it to fail a save are probably 1 in 20. (Actually, assuming a starting 16 int, a +6 headband, and 4 level increases Greater Spell Focus: Necromancy, the Horrid Wilting DC could be 30. Which means there's actually a 50% chance the Pit Fiend could fail. So with Greater Spell Focus and Greater Spell Penetration, the best a wizard can manage is a one in ten chance of dealing significant damage to the Pit Fiend with direct damage magic).
So wizards will need to do other things against it. But other things like what? If the current Summon Monster charts are kept, there's nothing that will even be able to hit the pit fiend's armor class--let alone get past it's holysilver DR. (Unless the PC summons another demon or a devil of course). So evocations are out. Summonings are out. About the only thing left is buffing fighters with mass haste, etc. But how will they do against it?
16th level fighter defense focussed: attack bonus +30/+25/+20 (+16 BAB +8 str (26 str-16+3 (level)+1 inherent+6 belt) +5 weapon +1 weapon focus).
Damage: 1d10+15 +2d6 vs evil (+5 Holy Bastard Sword) avg 26.5/hit
AC: 46 (10+7 (+5 lg shield) +13 (+5 fullplate of moderate fortification) +5 amulet of natural armor +5 ring of deflection +4 Haste +1 dex +1 dodge feat)
Granted, this is something of a best case scenario. Even then though, the fighter is only likely to do 42 points of damage per round. And, while the Pit Fiend isn't too likely to hit him (requiring a 16+ to hit) it's got much worse things to do to him like quickened mass hold monster+Coup de Grace (hey, he's Eevil).
How about an offense focussed 16th level character?
Attack bonus +33/+28/+23 (+16 BAB +11 str (str 18+3 level +1 inherent+6 belt +4 rage) +5 weapon +1 weapon focus)
Damage: 1d12+23+2d6 vs evil (+5 Holy Silver Greataxe)
AC: 44 (10+5 (+3 animated large shield) +10 (+5 mithral breastplate) +5 amulet of natural armor +5 ring of protection +4 dex +1 dodge feat +4 haste)
This fighter is likely to do 68 points of damage per round. Still, the meteor swarm will take him out before he beats the pit fiend--assuming the quickened mass hold monster doesn't do it first.
The party's cleric might send him packing with Holy Word but faces the same spell resistance problem as the wizard--he needs to roll a 16 in order to touch the creature (and that's with Greater Spell Penetration). He might ward the party with Holy Aura too. But that's unlikely to do any good. SR 25 won't help much when the thing's caster level is figured in (probably at least 17 considering the spells it tosses). And its other effects won't beat the Pit Fiend's SR. When summoning monsters, the cleric suffers the same problem as the wizard. . . except he can't even summon the demons or devils that might get past its DR.
So, assuming that the rogue is a similar damage dealer to the defensive fighter (unlikely as rogues won't be hitting AC 40 easily) and that there's an offensive fighter in the group and that both of them make their saves against the fear and the mass hold monster and that the meteor swarm doesn't kill either of them for some reason, they might just kill it by round 3 or 4. However, I can't see that fight as being 20% of their resources. It sounds a lot more to me like a tough fight or even "pull out all the stops because otherwise you'll all die."
In essence, the rules change made the creature tougher--certainly more than CR 16--but much less interesting to fight since it's really only vulnerable to one or two tactics. I'm glad that outsiders are getting more hit points. They've always gone down like chumps when I've used them. However, increasing their SR, to unbeatable levels and pumping their AC and physical attacks etc too is likely to make them uninteresting creatures to fight.