That Emperor 400 is ten times the filter as that Whisper 60. I use Emperor filters (280's and 400's) for everything. They are phenomenal. For the time being, you've got a great setup. You're issue at this point isn't gonna be filtration though, it's going to be the volume of water that that Oscar is living in. Eventually, if all goes well, it's going to outgrow your tank anyway.
Which filter did you just buy? An E400 will take a few months to actually build up enough bacteria in the wheel before it's fully operational. A that point the filter will be almost bulletproof, so long as you don't end up doing a massive water change at some point and kill off the wheel with chlorine. That W60 on the other hand isn't much of a haven for a bacteria colony. Even if you've got the one that comes with the sponge after the filter cartidge. The issue is that Oscar waste just seems to work it's way though the cartridge and muck up the sponge. When that happens your water just ends up pouring out of the filter right over the intake tube and never passing though the media. If the sponge is clogged there really is not gentle way of cleaning it out an sparing the bacteria colony within. The W60 is a good filter for a hospital tank though.