The book Optimizing Windows NT should be your first source for information on tuning and optimizing Windows NT.
The SizeReqBuf parameter is the only one that you might want to change to improve network performance. See Appendix B, "Registry Value Entries," in Optimizing Windows NT for information about this parameter.
Windows NT includes PMTU detection that automatically determines the optimal segment size on a per connection basis. Therefore, you don't need to edit the Registry to set the MTU size for the network interface(s).