Good timing. I just got my latest issue of Cycle World which had a liter-bike shootout. Nine sportbikes, all 1000ccs. Two V-twins (Ducati 1198S Corse and KTM RC8R), one V-four (Aprilia RSV4 Factory), and six inline-fours (BMW S1000RR, Honda CBR1000RR, Kawasaki ZX-6R, MV Agusta F4, Suzuki GSX-R1000, and Yamaha YZF-R1).
Interesting.
I know squat about bikes, but maybe the fact that these are all "sportbikes" and (I assume) all tuned to get the very most out of their available displacement, then the differences (if any) resulting from number of cylinders in minimized? Or put another way - in a less maxed-out state out tune (such as one would expect in volume-selling motor vehicle), is it perhaps easier to get more power from an engine with more cylinders but the same displacement? Again, I don't claim to know.
It's just my perception that, over time, 2.5l V6s seem to make more power on average than 2.5l 4-pots - but there are lots of variables at play. Only 0.2l separate a 3.8l V6 from a 4.0l V8 - but I expect the output of the latter to be higher. That may be an unfounded expectation on my part, but there you go.
Jaeger
Yep, these are all track-oriented sportsbikes, not sport-tourers or cruisers (cruisers are the two-wheeled equivalent of lowrider pickup trucks...don't get me started). All designed to be as fast as possible within the 1000cc displacement limit, the limit of the chassis, and the limit of the company's budget for the bike's development and manufacture (Ducati can get more out of 1000ccs with their US$70,000 Desmosedici, but these bikes are all in the $15,000-$25,000 range).
I don't think that the engineering limits of i4s versus V6s has much if anything to do with the cars we're discussing, because none of them are even close to the state of tune that a modern sportsbike is. That last point--budget for the vehicle's development and manufacture--is why I think you tend to see more performance out of 2.5L V6s than 2.5L i4s...because the 2.5L V6s are more expensive engines so the company can spend a bit more time tuning them.
Think about it. A base Altima or Fusion is a cheap vehicle. The engine would similarly be very inexpensive. A Spec V or Civic Si or Accord EX has a higher MSRP, part of which can go to engine development and manufacture, so the companies can extract more power from their four-bangers. Cars like the IS250 and G25 are a LOT more expensive, so the car companies have a lot more funds to dedicate to engine development and manufacture, and they end up getting a lot more out of their 2.5Ls.
The ONLY car we've mentioned that doesn't fit well into this three-tier budget discussion is the new Sonata, and I think that's just a matter of it having DI, which boosts power by about 25 horsepower without a lot of extra cost, and its competitors don't have DI yet. Once the rest of the manufacturers update their designs with DI as well, I suspect that the "cheapo" 2.5L i4s will approach 200 horsepower, the "upgrade" i4s like the Spec V and Accord EX will approach 220 horsepower, and the premium 2.5L V6s will approach 240 horsepower, and the three-tiered analysis will remain valid.