I believe that those larger engines with larger carbs may be less suseptible to these types of problems than the smaller engines that I normally run. My larges is a 120 4 stroke (I have two).

Still - it isn't pressure that is the demon here it is gravity. If you have two containers, one higher than the other and a tube in between them connected within the level of the fluid of the upper container. Gravity will force the fluid out of the top container until it either reaches a level of equality or the fluid level runs below the tube. In out case the higher container is the tank and the lower container is the carb. Now, in some engines - they need a pump to draw and this gravity can actually help things but up in the air when things are sloshing around and moving inverted the lack of pressure in the tank and drawability from the carb will give poor performance results. Take for example an OS 160 2 stroke. I've got a buddy who has one and it just doesn't draw well; I believe the carb is too big allowing atmospheric pressure to be more dominant than internal vacuum pressure within the system. A pump fixes this.

The quality of the carb can also impact things - I tend to buy used (though I have bought the occasional new when times are good) OS engines. I like their quality and longevity. I also buy Magnums because of their overal value. Good running engines with good life and parts that are easy to come by. Plus - global is pretty local to me and I have a good relationship with some folks there. The OS carbs are leaps and bounds higher quality in my eyes than the Magnum carbs and their engines tend to run better to boot. I have an OS 70 and a Magnum 70 4 stroke. Carbs are different, probably a lot of other differences too but they both perform well enough. The OS however is a power house - has almost as much power as a 91 - in fact I get nearly the same RPM with a 14x6 MAS prop as I do on my 91s.

Still, that OS 70 SII does not like to have a fuel tank that is too high.

There are a lot of different factors that can impact engine reliability and performance from design of the engine, tank placement, needle adjustments (duh!), fuel, plugs, etc...

With so many things that can be wrong - I just don't understand why these plane designers compromise and put tanks in positions that are higher than the level of the carb?

Regardless of your individual results you can't argue with 'best practice' and I think we can agree that 'best practice' is to put the tank on the level of the carb. In the Venus (my ongoing example) the tank center line is a a full inch above the center line of the carb. If you take a look at all of those beginner books that describe these sorts of things they tell you where the tank should be. Yet these guys insist that tank placement is not that important.

I've ranted and raved about this till my keyboard is blue in the keys but I guess the point I am trying to make is that I don't understand why they do it 'wrong' to begin with when it is so easy to provide a better product and do it 'right'.

Your mileage may vary.