I do not have the performance numbers, but 300Mbit/sec is not the true question.
We're looking at connection/sec.
The ios slb works great when traffic is loadbalanced without changing src or dst ip addresses. As soon as you nat, everything is software switched and performance drops.
So, for HTTP loadbalancing I would say you should look for a loadbalancer.
The CSS11501 is the smallest one and can achieve what you need.
If you expect to grow the amount of traffic, you can consider the ACE module.
Gilles.