Is this actually faster, i.e. have you run `make bench-net bench-http bench-tls` before and after?  The reason I ask is because I chose the stringification approach deliberately.

When you sock.write() a buffer, node.js has to retain a reference to the buffer until the write completes.  Strings don't have that issue.  They do require a malloc() and a memcpy() but so does Buffer.concat(), effectively.

I don't object to the change (provided it's faster) but I suggest that you replace the call to Buffer.concat() with manual buffer allocation and copying.  OutgoingMessage#write() is hot code so it should avoid the temporary array and the for loops that Buffer.concat() does over its elements.