There are two problems:

1. The form of anynomous function is icon(function($value,$row) {...}). So you the $row is in second parameter.
2. The $row will return the original data only so "calclatedField" is not available. You need to re-calculated inside the icon(). or better you create a stateless calculated function inside table:

class MyTable extends Table
{

    public function calculateField($row) {
        ... You calculation
    }
}

and then you can do:

Calculated::create("calculatedField",function($row){
    return $this->calculateField($row);
}
->label(""),
Icon::create()
                ->label("")
                ->icon(function($value, $row) {
                    $calculatedField = $this->calculateField($row);
                    if ($calculatedField == "Low Performer"){
                        return "fa fa-exclamation-triangle";
                    }
                    elseif ($calculatedField == "Average Performer"){
                        return "fa fa-balance-scale";
                    }
                    elseif ($calculatedField == "Strong Performer"){
                        return "fa fa-dumbbell";
                    }
                    elseif ($calculatedField == "Top Performer"){
                        return "fa fa-medal";
                    }
                })->cssStyle("font-size:28px"),

Hope that helps