Have you read the thread on filter media setup?
viewtopic.php?f=9&t=30145#p288911Particularly the top which talks about why you would want to set things up differently for the bioload of a turtle? Although, the only real difference between the Xp3 guide and here is not using the micro filtration pad and putting the carbon first.
I don't own a Rena filter, so this is pure supposition. First, you don't want to change everything at once. If you do, you lose your beneficial bacteria. That said, I would guess the sponges are probably clogging up faster and are harder to clean. You might try changing out two each month over two months. That may get you some improvement. From what I've read, the ceramic biomedia should never need changing, but I've only had mine for nine months. Do you rinse that in aquarium water when you clean your filter?
If you do decide to change that, again, you want to stagger the changes.
EDIT: have you cleaned or replaced the hoses. Over five years, you may have quite the buildup in there, which is quickly gumming up the rest of the filter.