Internet data traffic is doubling each year, yet bandwidth does not appear to be growing as fast as expected and thus short falls in available bandwidth, particularly at the "last mile" may result. To address these bandwidth allocation and congestion problems, researchers are proposing new overlay networks that provide a high quality of service and a near lossless guarantee. However, the central question raised by these new services is what impact will they have in the large? To address these and other network engineering research questions, high performance simulation tools are required. However, to date, optimistic techniques have been viewed as operating outside of the performance envelope for Internet protocols, such as TCP, OSPF and BGP. We dispel those views and demonstrate that optimistic protocols are able to efficiently simulate large scale TCP scenarios for realistic, network topologies using a single Hyper-Threaded computing system costing less than $7000 USD. For our real world topology, we use the core AT&T US network. Our optimistic simulator yields extremely high efficiency and many of our performance runs produce zero rollbacks. Our compact modelling framework reduces the amount of memory required per TCP connection and thus our memory overhead per connection for one of our largest experimental network topologies was 2.6 KB. That value was comprised of all events used to model TCP packets, TCP connection state and routing information.