During TemplatedStrings work Jim noticed that we could probably profit from reuse the same table lookup mechanism we're using for the rightmost bytes also for the 1 or 2 leftmeost bytes:

        // We know there are at most two digits left at this point.
        buf[--charPos] = DigitOnes[-i];
        if (i < -9) {
            buf[--charPos] = DigitTens[-i];
        }

This avoids some arithmetic, and the tables are likely to be cached. A small improvements on microbenchmarks, including Integers.toString, can be observed.