The thing is, <visible_on_cart> and <visible_on_product> are in Paypal's config.xml, and if they exist as a property, __get will not be called and a null value will be retrieved every time. But since the __get method is setting them dynamically, I think there's no way around it other than to refactor the method.

An alternative approach could be to use array instead (untested):

     public function __get($key)
     {
+        if (array_key_exists($this->_config, $key)) {
+            return $this->_config[$key];
+        }
+
         $underscored = strtolower(preg_replace('/(.)([A-Z])/', "$1_$2", $key));
         $value = Mage::getStoreConfig($this->_getSpecificConfigPath($underscored), $this->_storeId);
         $value = $this->_prepareValue($underscored, $value);
-        $this->$key = $value;
-        $this->$underscored = $value;
+        $this->_config[$key] = $value;
+        $this->_config[$underscored] = $value;
         return $value;
     }

@elidrissidev your solution is much better, you want to commit it or should I do it?

I won't have the chance now, you can do it if you're free.

I can never make a sandbox paypal account work but I've reworked this PR, I thought to check _config also for the underscored key... what do you think?