I've used the MACS Muffler on a .46LA and liked it, but it doesn't fit well on many planes with upright or inverted engines, as the wing is in the way. Fits great on a profile, but might be in the way of the LG. I had mine on a Humongus and put a little bend in the tailpipe to make it fit over the wing. Scott Riese did a bunch of promoting this pipe setup and said the launch rpm had to be 10,200 rpm. My experience showed that was good information, but would say perhaps 10k > 10.4k would get it working. When it's working, the engine sort of rattles like the muffler is falling off, but it isn't. It's hitting the right rpm in the air is all.
I found the 11.5 x 4 APC worked good on 10% nitro, but I also experimented with a cut down 12.25 x 3.75...to about 11.5" IIRC. I put that engine/MACS muffler on a Forerunner and flew it in Sparks, NV at 4,900' and it didn't work...couldn't get it up to 10,000 rpm. I had two options, run YS20/20 or change to a 10.5 x 4.5 APC, and chose the latter. It worked fine. That was 2007.
In 2009, I went to VSC with an ex-Don McClave short winged Skylark that I put the same .46 in, but with a Randy Aero CNC tube muffler. I had no complaints about the engine run quality with the 11.5 x 4 APC/10%, except that the tank was too small, even at Tucson's altitude. I changed to a .272" bore venturi (.156" spraybar) but it was still marginal. I had drilled out the exhaust tube bigger but also added a rubber ducky to deflect oil downward. I'd do that on a Classic again in a heartbeat. If Randy has those available again, that's a good muffler, but then you really need the rubber ducky and they're not light. The Jett muffler also needs a rubber ducky. The Scott Dinger muffler would not and is your lightest option. If you get ahold of Scott Dinger, ask about getting it polished and anodized...let him do it. No idea what the $$$ is for that optional extra.
IMO, the OS .46 LA muffler is perhaps a bit restrictive. I've watched guys flying ARF Pathfinders using the stock .46 LA with the stock muffler and the "wonder prop", 12.25 x 3.75 APC. They ALWAYS seemed to be suffering from being either too lean or too much prop. Perhaps more nitro or bigger venturi would have helped, or maybe that "10%" was really back in the day of it being actually 7%. You can always put a sleeve in the exit to return it to stock size.
I used a stock .46 LA muffler on my Magnum XLS .36 and that was a sweet combination...but I also needed the nose weight. One option would be to use the Magnum .36 muffler on your .46...it is smaller and lighter, but might need to have the tailpipe opened up some.
Steve