Never used Spa Purge but have used Swirl Away and it does the same thing. It does something but I don't know how much ... of coarse it can't clean something that may not be there. It also may be my imagination but it seems that my swirly jets work better after using the product.
Have you soaked your filters in a filter cleaner vs spray cleaning them? They seem to come out better than spraying them, just make sure you really rinse well. Also, I have soaked my filters in bleach to clean and they came out pretty nice.
As far as replacing them, my dealer said about every 3 to 4 years since I had 2 sets.
Be careful about Leslies, some of their products are less potent than other brands but if you get a good deal on them then they are great for chemicals - I buy most of my pool chlorine from them.