Execution order vs script insurance

I've got a custom input manager. I already know that the optimal solution would be to configure it in the execution order. But I'm asking whether the overhead penalty is huge enough to be considered problematic.

Alternatively, is there a way to "auto-add" a script to the begining of the execution order via the script iteself?

Ideally I want this to be a drag and drop script and work 100% script.

Here is the code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 /// <summary>
 /// DO NOT USE INPUTS IN LATEUPDATE WITH THIS INPUT MANAGER
 /// Only handles buttons - use default Input Manager for mouse movement and joystick movement
 /// </summary>
 public class MyInputManager : MonoBehaviour
 {
     #region SingletonMethod
     private static MyInputManager _instance;
     
     //This is the public reference that other classes will use
     public static MyInputManager instance
     {
         get
         {
             //If _instance hasn't been set yet, we grab it from the scene!
             //This will only happen the first time this reference is used.
             if(_instance == null)
                 _instance = GameObject.FindObjectOfType<MyInputManager>();
             return _instance;
         }
     }
     #endregion
 
     public AxisElement[] Axes;
 
     private Dictionary<string, float> Axis = new Dictionary<string, float>();
     private Dictionary<string, float> AxisPrevValue = new Dictionary<string, float>();
 
     private bool ranUpdate;
 
 
     private void Start()
     {
         foreach( AxisElement ae in Axes )
         {
             Axis.Add(ae.name, ae.axisValue );
         }
     }
 
 
     private void Update()
     {
         if(!ranUpdate)
         {
             foreach( AxisElement ae in Axes )
             {
                 
                 AxisPrevValue[ae.name] = Axis[ae.name];
                 Axis[ae.name] = 0f;
                 foreach(KeyCode kc in ae.PositiveKC)
                     if( Input.GetKey( kc ) )
                     {
                         Axis[ae.name] = 1f;
                         break;
                     }
                 foreach(KeyCode kc in ae.NegativeKC)
                     if( Input.GetKey( kc ) )
                     {
                         Axis[ae.name] = -1f;
                         break;
                     }
                 
             }
             ranUpdate = true;
         }
     }
 
     private void LateUpdate()
     {
         ranUpdate = false;
     }
 
     public float GetAxis(string identifier)
     {
         Update();
         if( Input.GetAxis( identifier ) != 0f )
             return Input.GetAxis( identifier );
         else
             return Axis[identifier];
     }
     public float GetAxisNoUniMan(string identifier)
     {
         Update();
         return Axis[identifier];
     }
     public bool GetButton(string identifier)
     {
         Update();
         return Axis[identifier] != 0 || Input.GetButton( identifier );
     }
 
     public bool GetButtonDown(string identifier)
     {
         Update();
         return ( Axis[identifier] != 0 && AxisPrevValue[identifier] == 0 ) || Input.GetButtonDown( identifier );
     }
     public bool GetButtonUp(string identifier)
     {
         Update();
         return ( Axis[identifier] == 0 && AxisPrevValue[identifier] != 0 ) || Input.GetButtonDown( identifier );
     }

}

The script works 100%, does not require any adjustment and inputs can be changed on the fly.