Hi,
I think the ~25% weekly change is usually advocated because that's what people generally find is needed to keep their ammonia and nitrite at 0ppm and their nitrates under 20ppm. It is my understanding that too large a water change (esp over 50%) could risk causing a mini-cycle / loss of too much good bacteria in the process, although someone else may be able to confirm if this is the case. If you were finding difficulty in keeping water at the levels of ammonia and nitrite at 0ppm and nitrates under 20ppm, then larger and/or more frequent water changes should address that, so that's why it's so important to monitor those weekly just before a water change so that you know what they tend to be - and monitor more frequently if you make any changes to the tank such as add fish, feed something different, do smaller/larger water changes, add/remove filter media, etc.
By biofilm (which is generally good ie the good bacteria which is on the internal sides of the tank and on the decor), do you actually mean the slightly oily look on the water surface? If so, this is more likely a build-up of fats (etc) from the food. If you don't like it, you could always do two water changes per week.
Be careful about changing the filter media - filter companies often recommend this despite it being unnecessary, and it can induce a mini-cycle (and therefore ammonia, nitrite and nitrate spikes). Although it does recommend splitting those, the key rule of thumb with changing filter media is not to change more than 30% at a time and generally to leave about a month between any further changes to give the good bacteria a chance to re-grow/build up. If the sponge isn't falling apart (just stained), then I would not change it but simply just gently squeeze it in old tank water, to avoid losing too much bacteria. If/once it's at the stage of falling apart, then you could consider cutting it and removing/replacing a third of it, then perhaps in a month's time, removing/replacing a third of the ceramic media, etc.
If you let us know what filter you have, though, we may be able to advise further and more specifically. I have the Marina i110 filter (take a look at it to see if it's similar to yours), and it only allowed the complete replacement of the cartridge, not a partial replacement, so I ended up converting it over to purely sponge media which has made it a lot easier. If yours is similar, I can advise in more detail how to go about that.