Not sure what size, just kinda winged it. Most of my muff knowledge is from car and motorcycle stuff, you definitely need some backpressure, I don't think it's a problem but in hindsight I would've only gone with one hole per tube or used a smaller bit at least to start
Okay used it this morning for 45 min or so. WOW. It is shocking how big of a difference the muff mod makes. Tap the throttle and it just brapps right up. It's louder but not obnoxiously so. Sounds super mean