You'll be fine with 2 405s like everyone else is saying. If by chance you do happen to upgrade to a bigger tank, then you can upgrade your filters to a Fx5 or 2.
Why? The bio filtration is working great. He has pristine water, so why add another filter? The bacteria will need more waste to process, to colonize another filter... So if the FX5 is added along with the 2 405s, you'll need a bunch more fish, so that it can all be colonized with the required number of bacteria.... If your nitrates are under 10ppm you have enough filtration. Dead serious.