Would I run into issues using softPWM on that many channels then?
I'd guess so, depends a bit on the base frequency you need to have with your PWM channels and the resolution it should have. But a frequency of 1kHz and 8bit resolution (15 channels) is probably more than the ATmega328 can offer.
I could imagine lots of extra wiring and loads of PCB's made if i'd need a WS801 IC per every 3 LEDS though smiley-sad
The WS2801 is integrated into a lot of LED strings available in many online shops. If you wanna do it centrally, there is the MAX7219 chip with lot's of channels or the TLC5940 which may drive a whole strip of about 10 LEDs with its 120mA.