As far as I know, the autothrottle on a large aircraft is FAR slower and more sluggish to react to a change in speed than a pilot controlling the speed on manual thrust.
The 'as far as I know' part, is the problem.
It appears that you might be just a tad misinformed.
On the Lockheed tri-motor, just as one example, the autothrottles are very fast acting so...Vref+5 is quite satisfactory.
And, as this is a rather older design, I'm quite sure newer (especially Boeing) types would be just as good, or better.