I've heard it as well, but it's not due to the oil pump (don't know if it's different or not but it's not relevant). The issue is the built in oil/water (engine coolant) oil cooler that mounts to the block where the spin on conversion would normally go. The regular 2.3L doesn't have this oil cooler; Mazda included it to provide some additional protection for the oil due to the stresses created by turbocharging.
It can be removed and the coolant lines plugged so that the spin on parts can be installed, but then you'd lose the cooling protection. IMHO it's not worth it; keep the oil cooler. The cartridge system is a bit more complicated but it wasn't that bad, if you have the right tools. The splash guard/airflow smoother was the most work, and that will still have to be removed and installed whether it's a cartridge or spin on.
It's way better than the cartridge system in my old '87 Acura Legend, which had a spring loaded aluminum cover held on my 3 small acorn nuts. Undo them and oil would spill all over, the nuts would drop and get lost while the cartridge falls out and splashes more oil everywhere.
Update. according to another member, who says he's done this conversion on a MS6, spin on filter adapter from a Mazda Tribute will work. The 2.3 in the Tribute came with a built in oil cooler on the spin on adapter. Hopefully he can post the part numbers. Here's the thread in the MazdaSpeed6 forum: http://forum.mazda6club.com/index.php?showtopic=59831