I think the next one up is the 600, which funny enough does 600 l/hr as opposed to the 400! I think the powerhead mounts are supposed to fit most of the models anyway.
The only thing to be careful with the Juwels is that I beleive the theory behind the Juwel filtration system is that water is supposed to be drawn slowly through the filter media. So, you will increase the output by upgrading, but I'm not sure it actually benefit the filtration as such. I think it's fine for tropicals, but if you've got goldies in there you may an additional filter to get your 10 x filtration.
Is this the tank with the external filter set up or is that the new one you have?