yeah, I did that with a eclipse tank - glued two cartridges together, and did some cutting. Took half an AC200 foam piece. I coulda fit the entire piece, but I wanted to leave some room for other filter media...
I think making a place to hold a sponge is the single best thing to do to a Penguin, but they still don't have the media volume and versatility that an AC has.
I can put in two foam blocks, and still have room for a bag of whatever I want. I'm also sure it has good water flow through the media when the box is stuffed full because AC firts routes the water underneath the media, then up through it. Stuff the box full on a Penguin, and the lower half just won't have any water moving through it (without further modification).
You can also very easily adjust the flowrate on the AC. Even cooler is that when you turn down the flow, it recirculates the water within the filter so the water gets filtered just as much.
I also really hate the impeller design on Penguins/Whisper. Once the little rubber things on the end of the impeller get old and hard, it's a pain to get that assembley put back together right. Having the spindle fixed in the motor like AC does is much easier for filter maintenance.
You can also easily adjust the length of the intake tube - they just press together (and you can get extra pieces for really deep tanks) as opposed to penguins one-piece design.
Don't get me wrong - Penguins are fine filters, but I just preferr the versatility and utility that AquaClear gives you for basically the same price.