Actually, an aftermarket clutch can have anywhere from 0% additional holding power [equal to stock] clear up to 150% more holding power than stock [1 1/2 times the clamping force of the stock pressure plate]. By changing what friction material the clutch disk uses along with the clamping force, you can get a clutch that holds more power under more abusive conditions [hard shifting and launches, and/or just a lot more engine power], yet can still be gentle enough for daily driving in the city [My ClutchMaster's Stage 3 kit comes to mind for this].

Lighter flywheels reduce the moment of inertia of the engine. This means that less force is required to spin it up to the same RPM of a heavier flywheel, but by the same token also means it has less rotational inertia. Lighter flywheels show thier gains after the car has started moving, but doing so requires a higher starting RPM due to the lack of inertia a heavier flywheel has [resists slowing down as much]. Also since a ligher flywheel has less inertia, it means that it will lose RPM between shifts faster than an engine with a stock flywheel. The skinny of it is that the engine will both gain and lose RPM faster with lighter flywheel.

And yes, it is generally a good idea to change both, especially if you plan on changing the flywheel. While you could change the clutch without changing the flywheel, I would suggest strongly against re-using a clutch on a new flywheel.