Ice Cap 48XL Sump

I purchased this sump a number of years ago. I'm just in the process of finally putting together my aquarium. I'm wondering if it would be okay to open the ATO section somewhat, so the return section can become larger to accommodate a RD3 Speedy 150-230W DC Pump. The sump is being used for a 240 gallon Freshwater aquarium. I will not be using the ATO section for the intended use of storing water for evaporation. There is no skimmer to upset either. Please let me know if I'm opening up an issue with the intended use of this sump. Thanks for your time.