You can kinda see it on DB99's pic, the painted cill on the Golf has a very slight raised ridge, ie. its not completely flat, although having checked my own just now, its not that far off being flat.
If the Polo's cill plate doesn't account for that slight ridge (if the Polo doesn't have the same ridge), then it may not sit perfectly on the Golf's cill, but I reckon it would be damn close.
Apart from that, I can't see why they wouldn't work fine.
Someone with access to a Polo could compare the cills with those of a Golf. It doesn't need to be a GTI Polo.