Question about shader optimization (rendertarget to 1x1 texture for data)

texture colorTex : COLOR;
sampler colorSamp { Texture = colorTex; };

// x = left or image without black borders, y = top, z = right, w = bottom
float4 CalculateViewport()
{
    float4 viewport = float4(0, 0, 1, 1);
    
    // Look through the pixels on x and y axis until one that is not black is found.
    while (dot(tex2D(colorSamp, float2(viewport.x, 0.5))) > 0 && viewport.x < 1)
    {
        viewport.x += BUFFER_RCP_WIDTH;
    }
    while (dot(tex2D(colorSamp, float2(viewport.y, 0.5))) > 0 && viewport.y < 1)
    {
        viewport.y += BUFFER_RCP_HEIGHT;
    }

    // Let's asume the black bars are mirrored across the screen
    viewport.z = 1 - viewport.x;
    viewport.w = 1 - viewport.y;

    // Return our gathered information. It's a rectangle containing the position and size of our image without black borders.
    return viewport;
}

texture viewportTex { Width = 1; Height = 1; Format = RGBA32F; };
sampler viewportSamp { Texture = viewportTex; MinFilter = POINT; MagFilter = POINT; };

float4 CalculateViewport(in float4 pos : SV_Position) : SV_Target
{
    ...
    return viewport;
}

float4 SomeOtherShader(in float4 pos : SV_Position, in float2 texcoord : TEXCOORD) : SV_Target
{
    // Retrieve our viewport information from the texture we rendered it to below.
    float4 viewport = tex2D(viewportSamp, float2(0, 0));

    // Lets just use it to discard/abort rendering in the black borders and only do stuff on the actual image.
    if (texcoord.x < viewport.x || texcoord.y < viewport.y || texcoord.x > viewport.z || texcoord.y > viewport.w)
    {
        discard;
    }
    
    float4 color;

    ...

    return color;
}

technique MyTechnique
{
    pass
    {
        // Render the result of the CalculateViewport function into a texture ("viewportTex") only once (at start of post processing).
        VertexShader = ...;
        PixelShader = CalculateViewport;
        RenderTarget = viewportTex;
    }
    pass
    {
        // Now we can execute other shaders and use the result stored inside "viewportTex".
        VertexShader = ...;
        PixelShader = SomeOtherShader;
    }
}