string[] preallocation to prevent using ToString() and save memory

I have some levels, with enemy waves and two Text components for:

Enemies killed, like: "13/57" Waves passed, like: "68"

On each enemy kill/new wave, I make a ToString() call, in order to update these UI components. I already separated "13", "/" and "57" into different Text's to prevent concatenation.

In early game, there's no overhead, but if the game gets more dynamic in time and there are 30-40 enemies on the screen, GC get's mad really quickly. It's not a constant Update(){ tostring() } call, but it's noticable, really, especially while rendering.

The game itself is really, really expensive, so I need to optimize what I can.

The point

The maximum amount of enemies is 200, so this is the maximum string that will be made through ToString().

There's no maximum wave, but it's quite hard to get to 500 (late-game).

I made a simple system:

private static readonly string[] STRING_NUMBERS;

static MathUtils(){
    STRING_NUMBERS = new string[1000];
    for(int i=0;i<STRING_NUMBERS.Length;i++)
    {
        STRING_NUMBERS[i] = i.ToString();
    }
}

public static string GetStringNumber(int i)
{
    if(i > STRING_NUMBERS.Length - 1)
    {
        return i.ToString();
    } 

    return STRING_NUMBERS[i];
}

As you can see, I preallocated 1000 strings: "0", "1" ... "999".

If I try to get a higher number, it would ToString() anyways.

My question is, would it work? is this worth? Did I forget something?