The rule of thumb I've heard and used on my engine was around 500 HP. Its very application and tuneup dependant however. If the engine is detonating, it can hurt the bottom end at lower power levels and if everything is just right, higher power might be safe. Better safe than sorry though on the bottom end.

Studs put a higher clamp load on the bearing caps to help them stay put. They change the shape of the cap enough that line honing is needed when switching to studs.