Hi, IMO its 100% easy to do the sound mod in the slim vs the phat. The only thing you need to be good at is soldering some small contact points. Also its key to note that unlike the phat you need to drill holes in the Left and Right buttons because the triggers have thick whit plastic in the bottoms so leds can not light them from under the triggers. Also if you do the sound mod you will not need a 10ohm if you use the recommended LEDs..
Good luck and if you need anything let me know.