Setting the TcpAckFrequency may improve your apparent ping but will not improve your actual network speed. If anything, it may make it worse by increasing the number of actual packets being transmitted by your machine for the same amount of data.
When a computer connects to another computer using TCP it performs what is known as the TCP Three-way Handshake. what happens is this:
1. Client computer sends a SYN (synchronize) packet to the server computer
2. Server computer sends a SYN-ACK (SYN-acknowledged) packet to the client computer
3. Client computer sends a final ACK (ACK acknowledged packet to the server computer.
Thus begins the TCP connection.
All these SYN, SYN-ACK, and ACK packets carry no actual data. The Windows network stack, since Windows 2000 has, by default, only responded to every other TCP SYN packet unless additional data packets are not received within a specific period of time (per RFC-1122). This reduces the total number of non-data packets that must be sent. Changing the referenced registry value to 1 will cause Windows to respond to every SYN packet, thus doubling the TCP overhead for a connection. While this will likely improve the "ping" rating you get in online games (since it takes a SYN packet less time to be ACKed) it will not increase the actual speeds of data transfer but reduce it.
Microsoft recommends against changing this value without having a detailed knowledge of the network stack and how it interfaces with your applications.
Both the BitTorrent and FTP protocols run on top of the TCP protocol. Therefore, I would day that using this hack would likely affect them negatively. Gnutella clients like Limewire use a combination of TCP, HTTP, QRP, and UDP. UDP and QRP do not rely on TCP whereas HTTP and (obviously) TCP do. Thus I would say that Gnutella might be broken by this tweak.