These thoughts are based on my own experience of using the tweaks and what is described on microsoft's own site for what they do.
enablefastroutelookup - little known about this but when I have it enabled web browsing has weird pauses raondomly when loading websites so I now have it turned off.
BufferMultiplier - this usually gets reccomended on here to double to 1024, I noticed pauses when loading some sites occasionally when on 1024 so I set back to 512, from what I read on the documentation when sending packets it dictates the size of the initial packet so is effectively the base value for a slow start mechanism for when you uploading. Since on typical usage 99% of packets you send are smaller than 512 bytes it makes sense to keep this at 512.
MaxNumForwardPackets and NumForwardPackets - these basically increase the packet queue which might be good if you need to be able to handle more packets at once but does effectively add tcp latency, so if you someone like a WoW gamer or for some reason tcp latency is important for you then increasing these values is actually a bad idea.
MaxFreeTcbs and maxhashtablesize - these control the number of silmultaneous connections can do at once, the only typical usage scenario I can think off that these could help is when using torrents, the obvious downside of increasing the values is more kernel memory is used, I dont know if there is any kind of performance impact yet.
DefaultSendWindow - as reccomended increase this value, I personally set it to 32768, and buffermultiplier does not need to be increased for this to work, this tweak does also work on vista confirmed by me. It is basically the size of the tcp send window.
InitialLargeBufferCount and small and medium - this I expect simply creates more buffers and as such probably allows more concurrent connections, possibly will benefit torrents I expect no affect on other typical usage patterns tho, will increase kernel memory load.
EnablePMTUBHDetect - suggest disabling this in xp sp3 and vista, shouldnt need to disable in xp sp2 and below as defaults to off, its an alternate way of determining the mtu value developed by microsoft but in my experience its buggy and too conservative and slows performance. Probably the prime reason why people have tcp performance issues after upgrading to sp3 in xp.
I have some other tweaks I played with and comments on them and may post them later, please note these are my own personal thoughts and in no way I am saying I am correct in ym diagnosis.