How does the choice of vegetable ratio and processing steps influence the taste of mirepoix?
A bit late to the party but ....

Onion and celery have similar effect in the mirepoix, the leeks will have a similar effect but with more depth on the sweet side &amp; carrot will bring some sweetness.

Based on that, depending on what you want to cook, there are some guidelines. 


For stews, keep in mind that some of the next ingredients might add some sweetness to the meal, so it's ok to equal the onion and celery parts to the other veggies. 
For broth you might want to reduce the onion and celery part to 1/2 part of other vegetables (carrot, mushrooms, tomato paste, etc.) knowing it will eventually reduce into a demi-glace and that you will season it with spices and aromates. Reducing mean the flavor will concentrate. 
For soup / potage you should adjust to the main ingredient to balance it out, usually I go with onion and celery in equal part and no carrots unless you want the soup / potage to have a sweet taste, which is ok for pea soup, tomato, carrot, etc. 


Hope it helps,