Can I just use a huge RWIN value ?
Buffered data is obsolete, and may cause problems in the responsiveness of the protocol to changes in the peer state. Since data and requests share the same TCP socket, buffering lots of data means that your request for a block may have to wait a few seconds before it even gets transmitted. By this time the peer you are talking to may have decided that you aren't interested in his data after all, or your own client may decide that the peer you are talking to is snubbing you. Either of these situations has very significant negative effect on download rates.
RWIN is directly related to your bandwidth, and the maximum latency of your connection. To calculate the optimum RWIN value for your connection, read this FAQ, or simply use the SG TCP Optimizer recomended values.