I know there are dynamos with 3 W as the highest output, but are there commercial dynamos available with even better effect? See, I don't mind pedaling with a bit more drag if I can get better lighting on my bike.
Actually a 3 W dynamo is mislabeled. It isn't 3 W. It is 0.5 A.
A hub dynamo is mostly a constant current device. The faster it rotates, the higher is the frequency and voltage of the alternating current it produces. Also because a hub dynamo has coils which supply the electricity, the coils have an inductance. The faster the frequency of the alternating current is, the higher is the reactive impedance of the coil.
So while you pedal faster, you get more voltage, but you also get more reactive impedance. Both of these increase linearly, so if you short circuit it, you get approximately the same current at every speed.
Historically hub dynamos were used for halogen lighting by using a 6 volt 0.5 amp 3 watt bulb. However, you could wire up two bulbs in series for a 12 volt 0.5 amp 6 watt system. In this case, the higher output would be obtained at higher speeds -- exactly when you need the higher light output.
So if you want to get lots of light today, you don't need a higher-than-3-watt hub dynamo. You could use a LED light with intelligent switched mode power supply that can take advantage of the higher voltage produced at high speeds.
I'm not sure if anyone makes such a LED light, but it should be doable and it would approximately double the light output at high speeds. With a good enough intelligent switched mode power supply, it doesn't even need to sacrifice light output at low speeds. It could always operate at the maximum power point of the hub dynamo, similar to what MPPT systems do for solar panels.
Referring to the answers to an old question of mine one solution would be to run a hub dynamo and the best bottle dynamo you can buy.
Start with a standard hub dynamo setup with a top-of-the range light like the Busch und Müller IQ-X (I've seen a few, and want to upgrade to that from my Axa Luxx 70). That light is really good, with a wider pattern than many dynamo lights, but the top cutoff is too sharp if there's any risk of hazards above the beam, for example on off-road trails.
Personally, my additional lighting is battery powered; it also serves as a work light if I have a mechanical in the dark, can be lent to friends, etc.
But a good bottle dynamo (the Axa HR traction was recommended to me, or the discontinued Nordlicht) could be used to drive a second dynamo light, adjusted to give a higher beam, selected for a wider pattern, or both. The PedalCell rim dynamo/charger combination also looks interesting, combined with lights designed to be run off USB battery packs