Ameyam,

Ther is no an issue with running the 55AX inverted. The biggest issue is tank placement. If the tank is to high in realtion to the carburator it will cause fuel to run down into the carb when it does not need to. I have run a 55AX inverted several times in different planes and it is only a matter of taking the time to tune it properly.

As for the 4-strokes yes they do have more torques and most of your ideas are correct. The valve adjustment is very easy to do. The info can be found all over the internet on how to accomplish this. Most of the time you will only need to check the clearance. I have a old OS 91 4S that has never needed a valve adjustment in all thes years of flying. I just check it once a year or so to make sure it is holding its settings. As for props, the ability of the 4S to swing larger more efficient props is where they make up for their RPM disadvantage. They provide better throttle response on a more efficient prop whihc in turn creates amazing performance. I would probably use only 4S for 3D planes but the cost is just to much for me most of the time. $150 for a 55AX or $300 for a Saito 82. Not to hard of a choice for me.