Usually the Juwel filters work reasonably well on a tank with average stock levels, once they're matured biologically of course. They can struggle on heavily stocked tanks.
One option to upgrade is to go to the next pump size up, the 1000 LPH pump fits straight in to replace the 500 & 600 LPH models. Some of the foams can also be replaced with the Cirax ceramic media for more bio surface area.
If the tank will be really heavily stocked, e.g. a Malawi cichlid setup, then I would opt for a second filter, ideally a canister filter to run alongside the Juwel filter - the two should work well together. Or if you really prefer to remove the Juwel filter you could opt for a well oversized canister filter.