benchmark

$ go test -bench=.
testing: warning: no tests to run
PASS
BenchmarkBuffer      100          14674718 ns/op
BenchmarkPipe        100          14456488 ns/op

pipe_benchmark.go

package pipe

import (
        "bytes"
        "encoding/json"
        "fmt"
        "io"
        "io/ioutil"
        "testing"
)

const dataSize = 10000

func BenchmarkBuffer(b *testing.B) {
        m := makeData()
        for i := 0; i < b.N; i++ {
                var buf bytes.Buffer
                _ = json.NewEncoder(&buf).Encode(m)
                _, _ = io.Copy(ioutil.Discard, &buf)
        }
}

func BenchmarkPipe(b *testing.B) {
        m := makeData()
        for i := 0; i < b.N; i++ {
                pr, pw := io.Pipe()

                go func() {
                        pw.CloseWithError(json.NewEncoder(pw).Encode(m))
                }()
                _, _ = io.Copy(ioutil.Discard, pr)
        }
}

func makeData() map[string]string {
        m := map[string]string{}
        for i := 0; i < dataSize; i++ {
                m[fmt.Sprintf("key-%d", i)] = fmt.Sprintf("val-%d", i)
        }
        return m
}

If you move declaration buf bytes.Buffer out of cycle it will change a picture.
If you set dataSize to 10, it will change picture again.
I would stay with bytes.Buffer.