The thing that decides whether a manufacturer uses forged internals is mainly issues with cylinder pressures and the warranty. If there are high cylinder pressures, the pistons and rods will take a beating if they are not forged. Therefore, in order to not take a beating from warranty claims, they used forged internals. But, if they use forged internals, the tollerances must be much closer because the wear from the pistons no being able to expand like cast pistons will also cause high warranty claims. This is why the S2000 has some of the closest tollerances inside it's cylinders. They S2000 comes with forged internals because of the high rpms and the high compression. I've read that the tollerances inside the S2000 are pretty much the highest there is, not counting manufactureres such as Ferrari and Lambo.
Also a quick fact that most don't know about the S2000. The pistons speeds (speed that the pistons are traveling up and down), if that wasn't obvious, are the fastest of any car ever made. They are actually just a tiny bit slower than F1 cars piston speeds.