It's good to get opinions from professionals who can touch both worlds (Yamaha and Casio). From what I read Casio still manufacture cheap keyboards.
10 poly is very close to my PSR-260 (now < $100) which had 8 poly when I played the good quality voices.
As for my DGX-305. In some of my sequences there is polyphony drop when the keyboard plays < 16 notes simultaneously.
Anyway, it's too bad that the new casios are not true contenders to yamaha. A serious competition in the low-end market would force Yamaha to do significant improvements to the DGX line and make me feel interested in purchasing a new keyboard.
George