It might be advantageous to avoid using ByteString Builder altogether. If we keep a list of chunks from the input, we don't need to worry about the Builder allocating buffers which are just going to be directly written to the network.
go buffsize digest = do
mbs <- await
case mbs of
Just bs -> go (buffsize + length bs) (hash digest bs) ...
Nothing -> send (uploadPart & <to use hashedBody because we know hash and size>)
It might be advantageous to avoid using ByteString Builder altogether. If we keep a list of chunks from the input, we don't need to worry about the Builder allocating buffers which are just going to be directly written to the network.