I recently started switching my Hydro sponge filters over to Poret. I just bought the block foam and cut them to size. Eventually I'd like to move to Mattenfilters, but this was a good intermediary for me, and they've been working great.
I'm in the boat of using something that I know other people trust and have had good luck with. I don't necessarily like 'experimenting' with new fish equipment, same reason I go with name brand (recommended) heaters for tanks when needed.
In the end, it seems like longevity could be a key difference. I know you've said you have off-brand foam you've used without degradation, but I've never heard of anyone replacing Poret for durability reasons. Generally I follow the saying of "You get what you pay for." Its likely you'll spend less money now but may have to replace more often, or not get quite the same result.
Anyway, not trying to crap on your idea, just throwing out my opinion. I'm very curious to hear how your foam works out, if it lasts and performs, I'd be very tempted. One other thing to consider is customer service. I'm sure Stephan is available for questions or concerns and guarantees his product. Ordering some from amazon or ebay may not have the same level of service or guarantee. Best of luck though, post details of what you find.