I've read in a couple of articles to use a 15 W soldering iron when working on PS2 guts and chips. Is there any terrible reason why one should not use a 30 W iron ?
the 30w irons produce too much heat. When applied to the board of the PS2, it's not just heating that one surface. it's heating all of the leads from that point. Essentially it's just like your PS2 overheating. You wouldn't want your PS2 to overheat and fry itself. Applying a hotter iron is just the same thing.
If you're an expert and know what you're doing a 30w iron is ok as long as you don't heat the contact for too long.