Ok, I am one of the guys who answered your other post, I had the 55ax in a Funtana until my Funtana had a rather abrupt meeting with a Twist about two weeks ago. Here is what worked, period.

#1. This Dar Zeelon character who has answered some of your qusetions here...LISTEN TO HIM! I followed his break in method to a "T" and he knows what he is talking about. I now have one screaming ass 55ax due to following his procedures for break in.
#2. You really need to use a fuel that has some Cator in it, I use Morgan Omega as it fits the bill perfectly. Yes, I use Wildcat CY 30% all synthetic in my heli's as that is what they like, but in a cowled ABC type ringless engine you NEED the protection that Castor gives you. I will admit that my 55ax has some brown castor varnish on it, most likely due to it getting a bit warm while hovering even though I run it a bit rich, but it still runs like a bat out of hell because that castor does its job when the going gets hot. I think that my main needle was at around 2.5-2.6 turns.
#3. For the Funtana you might want to try an APC 12.25x3.75 (worked the best for me) or an APC 13x4W. I know that some people say to load this particular engine down some, but that 12.25x3.75 turned 14,400 on 15% Morgan Omega and the verticle performance was outstanding!
#4. I tried a pitts style muffler as have several others. I have only seen where one person got it to work seemingly well on this engine, but overall they have not worked well. I do not know why, it's just that way. I ended up using the $15 Tower 46 muffler to get my performance. I also picked up 900 RPM's over the stock 55ax muffler by doing so. To get it to fit I cut out a muffler tunnel where the firewall sloped down (you have the Funtana so you will know what I am talking about) to get it to fit. It was a little bit of extra work, but it works well. By the way, I tried a Du-Bro muffler extension on the Tower muffler to get some of the mess away and even that little bit of extension affected the engines performence so it does seem to be pretty sesitive to muffler back pressure.

I understand why you want to use the pitts style muffler, it just looks very clean on this airframe. That is the same reason that I tried it, but eventually you will have to decide between performance and looks, I chose performance. If you build the muffler tunnel though it does keep either the stock muffler or the Tower muffler pretty tight and it still looks good. I just decided to live with cleaning the airplane after flying, it is a bit of a mess at times, but I really wanted the ability to pull out of bad situation which it did in spades. On a side note I did get a bit better performance out of the stock muffler by pulling the baffle out, but it still did not compare to the $15 Tower muffler.