Engine speed (RPM) is a direct relation to horsepower. Those 4 bangers (actually 2.4 liters) scream from the factory. Something to the tune of 9,000 RPM if I remember correctly. Like Intensblu said those S2000s don't make any torque.

There is a lot of potential with these engines but as you mentioned the aftermarket is slim (but growing). All the ways you described are good examples to build more horsepower. There is another way, although expensive, to add quite a bit of ponies and that is the stroker kit.

Just imaging if there were intake manifold options, different cam shaft profiles to choose from, higher compression pistons, you know, pretty much what is available to everyone else - these cars would fly!

Oh to answer your question, the biggest claim that I have seen is from Renntech. Yours for the low low price of $14,900.