There does seem to be a line beyond which continuously manually tweaking the GC would be more work than manual memory management. GC allows you to get up and running faster, but the level of detail you need to know of the JVM to do manual tweaking of the GC implementation approaches the level of detail you need to be aware of in your app to do manual memory management.

That is to say, optimizing the JVM's GC settings and effectively writing/debugging C and C++ memory management seem to be wizardly-level tasks of about equal standing. TANSTAFL applies.