RB20s are extremely robust engines and can hold all the power. A bad tune, wearing out components, or a freak accident are what kill these engines. Factory sealed blocks are known to hold 350 - 400+ hp no problem. That being said, do that at your own risk. Many of these engines have been poorly taken care of and beat on for years. I would be cautions trusting a 30+ year old engine pushed well beyond its factory limits. A good inspection and a rebuild with quality parts is always my answer. 

Anyway, if you where to pick a weak point in the bottom end, it is the pistons. With extreme heat and boost you can have failures. Rods are soild as rocks. I really have not heard of anyone breaking rods at any power level. Torque is what kills rods and lets face it, there is none in these engines. My advice is to have a proper ECU with a really good tune and you will be fine. Forged pistons if you after bigger power or plan to abuse the engine and forged rods if you really feel the need.

The internals are solid, but I would highly recommend getting the block and head decked and running upgraded head studs and a upgraded headgasket. I personally have had 2 OEM headgasket failures at ~225 hp (unopened factory blocks) and just don't trust them. Many people swear by an OEM headgasket, but I say upgrade.