Hello Hoze 513.
I can offer some information about why the two types of chambers exist. There are some advantages to both types of designs. Both types have valid reasons for their use.
So, the center plug location can have an advantage in combustion efficiency due to the fact that it is located directly in the center of combustion activity. This is important because it is beneficial to complete the burning of the mixture quickly in order to reduce the chances of detonation to occur.
With that said, I'm not saying that it is impossible to design a slant head design which minimizes the problems of having a spark plug which is offset from the center of the combustion chamber. I believe it's just a little tricky, and a more complex combustion chamber shape is in order. In fact, I found the combustion chamber shape of the GT-5 head, when modified to incorporate a squish band had really good characteristics, but bad cooling, as you could expect as with all of the stock heads.
The slant head design has a distinct advantage over the center located spark plug hole in that it simply takes up less space. Some bike builders don't have much room above the engine for a center located spark plug design, and must use some kind of slant design.
So, how did I do? This comment is just based on my most up to date understanding about combustion efficiency, and why the two types of designs exist. I'm sure there are other things that I'm leaving out, but that's why we have this cool forum. It's kind of like a new kind of "open engineering" where anybody can actually impact how things are to be made.
-Fred