As above yes you'll notice a lightened flywheel but it's very slight and only in first second gear. And yes you will lose torque. There aren't really any major issues with lightened flywheels other than steer clear of cheap shit ones. As i've seen ones that have exploded(not in an s2000) and it's not a pretty thing to see.

O.k as for cams, where do i start. To put just cams and leave everything else untouched really is pointless, as the factory cams make great outright power. and most cam sets will only give you 5-10hp if your lucky. However if you were after more torque and mid-range power then a set of cams can do the trick, providing you find the right set.

If your after outright power, you'll need to add, prog. ecu, new valve springs(should replace these every 60-80,000 kms anyway), injectors, custom head porting and most tuners go to quad throttles. Then you'll need to go larger headers, 70mm exhaust and you should see a gain of around 30-50 hp gain depending on tuner and parts, but you lose low down drivability.

You need to ask yourself is it worth it?