Italian HP????

[fudge12]

Lemme start out by saying that I've never driven a Diavel.  The only ducati's I've driven were a Cagiva Elefant and my buddy's race 748.  That said, I'm somewhat experienced in the vagaries of different engine design.
V-Twin, harley style.  Single crankpin, dual cylinder. poTAto.  It's inherent in the design, and was originally designed that way not because it made sense mechanically but because it fit in the bicycle style frame they were using.
For those of you who made fun of buell competing with a big cc advantage, let me point you to the beginning of the end of 2-strokes where 250cc 4-strokes competed with 125 2-strokes, 450cc 4 strokes competed against 250cc 2-strokes.  Let me also point out that the buell was air cooled.  They're not directly comparable cc:cc .  If you want directly comparable then look at the CRF450 vs RVX450.  Single vs Twin, same displacement, nearly the same HP, but entirely different attitudes.
v-twin, split-crank pin style: by varying the split on the crankpin you can make any v angle act and or feel like any v angle you want.  Disadvantage here is crank wobble and weakness between the crankpins.
L-Twin:  Ducati, suzuki v-strom, honda vtr, aprilia, etc.  Perfect primary and secondary balance.  You can spin them like crazy and they don't vibrate.
I-4:  Lots of variations, from the big-bang to the crossplane.  Big bang fires two cylinders at a time like a pair of linked parallel twins, for discrete power pulses.  Crossplane fires one after the other after the other after the other.  Same power, different delivery, and different stresses involved.  Theoretically a crossplane crank can be made lighter since it only has half the torque applied per event.  More vibration though.
I2:  Typically considered buzzy, can be either 360* or 180*, with the difference being the same as crossplane vs big bang.
O2-4-6.  BMW, goldwing.  Perfect balance in nearly every way.  Seriously, youtube for 'the nickel trick'  Disadvantage is weight.
Ok, single or parallel vs twin, as defined by separate heads: This means anything from a single, parallel twin, I3, 4, or 6 vs a v-twin or v-4 (discounting the odd other style)  The more cylinders you add other than inline, the more weight increases relative to the displacement.  I4: 1 cylinder head, one or two cams, one cam chain, one water jacket/cooling fin set.  V2, two cylinder heads, 2 or 4 camshafts, two camchains or gearsets, two water jackets, etc.  Narrower but heavier for the same displacement.
Where do I bring useful info into this?  Who knows.  I've had both a vstrom 650 and 1000, (L-twin) and have their counterpart the Versys650. (parallel twin) I've had a VFR700 and a VFR800. (Both V-4, which are more accurately considered either linked parallel twins or linked L-twins)  I've had an XR400, XR416 and an XR650R and a buell blast.  (all of which are single cylinder thumpers) and I have a Dnepr MT16 I've got a GL1000 (Opposed twin and O-4)  Plus I've had any number of I4 bikes, not least of which is the FJ12.  Pretty much my only lack of experience is in I3s.  Any engine can be made to do nearly anything.  The 'character' a bike has is largely determined not by it's HP number but it's delivery of power.  The VFR8 has an awesome engine, quick to rev but torquey as well.  The Stroms made good power and were so smooth that revving them was irrelevant.  The versys has a much more sporty feel than the strom, but is still half the power of a modern 600cc supersport. 
To directly answer your question as to how the diavel can make so much power, look at it this way.  A cylinder only makes power for about a third of the crank's rotation, the rest is drag including the whole second rotation.  If you can get more than one cylinder to share that drag you can free up lots of 'extra' hp.  One cylinder fires, then the drag begins.  Before it gets too far, the other cylinder fires, it's power stroke overcoming the drag stroke of the first.  It's drag stroke begins, the first cylinder goes back to work and makes power again.  Gestalt, as it were.  More than the sum of it's parts.
Plus ducatis are built by hot Italian women, which inherently makes them faster.