I really don't have any comment on the inverted PItts muffler with the YS. I haven't tried it. I, just like Morbid and about 1 million other people, use a Saito 100 in my UCD. (The other million use the YS it seems).

I wouldn't bother using a high pitch prop on a UCD. That is not what the plane is meant for. I would suspect that flutter could be a problem with a high piched prop. These control surfaces are huge.

I forgot to mention that I'm using a 6V 1650mah NiMH battery so the torque I'm getting from my 3001's should be somewhere near 60oz. I went to 6V specifically for that reason. It also speeds the servos up a tad, although it's not really noticable.

As far as modifications, I made a few. I went with 4-40 pushrods on the control surfaces instead of the supplied 2-56 rods. I mounted my fuel tank about 3 inches back and used a cline regulator on my Saito. I did not use the stock tail wheel assembly because it felt cheap and weak. I used the sullivan instead. Other than that the plane is pretty much stock. You might look at beefing up the landing gear block as I've heard of a few people complain that it's weak. (Although I haven't done it). But for $189 the plane is a good deal. It's a big plane with loads of tricks up it's sleave. You'll like it.