all the guys i know fly the 150 DA with pipes. -i know someone who will fly the yak with pipes and 29x12,5 Delro 3 Blade Prop -that rocks -he ha same combination in the SX and it was great.
i wouldnt use Mufflers - not enough Power for IMAC -better to have more power to keep speed constant in up maneuvers
Chris
http://www.aerobatic.net.tc