If you look at Most Popular Ergonomic Keyboards, you will see that the buttons are almost always in a perfectly straight line.
The only thing that changes for the most part, is a split angle between the individual hands. Meaning, half of the keyboard is angled to match the angle of that hands arm.
A lot of arcade controller companies (Qanba, Madcatz, Hori, etc) tend to do this for that very reason.
Actually, Im fairly certain is was pretty much done for looks and lack of space. Japanese control panels are very short and space is cramped.
As for fingers, my index (2nd), and 4th finger are almost the same height when bent. Only the middle finger is slightly higher. However, that is solved because of the layout of the fingers and hand. The shorter fingers point towards the keys from a small angle, while the middle finger is directly vertically above the key. The result is that all fingers easily conform to the correct shape. This is even more so, when you are using buttons spaced apart in the typical Streetfighter 2 layout.
Edit:
Ok, I can see why you may want to drop only the leftmost buttons a little... and thats ONLY if you are using a 4button layout...
If you place your fingers on your keyboard, every other key.. Trying to span 4 keys at once is a task for the large handed, and even then, isnt the most comfy. By shifting the index finger (1st) one key below, keeping all the others in a straight line... the reach is lessened, and the angle of the wrist is more natural.
However, all that said, I would Never curve the rest of the layout. You middle finger will end up hitting the edge of the button, instead of the middle of the button, and its both uncomfortable, and at times, causes you to missfire.
Edit 2:
Posted Pic for reference.
Also, note that even with 6 button SF layouts, that there should be very minimal spacing between the buttons. Basically, you have almost no room to turn the button nut. Thats helps with wing-spanning all 3 buttons at once... especially if your fingers/hand is smaller... as well as keeping you from accidentally pressing dead space between the buttons.