Both items are great when they have some stacks, but when you buy them they have 0 stacks and are not really gold efficient.
Sword of the Occult is 30% gold efficient with zero stacks. Its gold efficiency increases by 15% per stack, becoming gold efficient at five stacks and reaching 404.3% gold efficiency at maximum stacks.
Mejai's Soulstealer is 35.2% gold efficient with zero stacks. Its gold efficiency increases by 14.1% per stack, becoming gold efficient at five stacks and reaching 354.9% gold efficiency at maximum stacks.
Buying them early is really risky because it puts you behind (you could have gotten something more efficient with that gold).
Should I only buy them when I know I'm already fed?
Edit: