Moving to Unicorn will probably not increase performance. The performance characteristics of Unicorn are similar to Passenger's. Plus, Passenger has far better administration tools and documentation, so that in the event of problems you can quickly figure out what's wrong.

Passenger works just fine with single-tenant dedicated instances. Just configure passenger_min_instances to the same number of max_instances or max_pool_size, and it'll behave exactly like Unicorn's default settings.