Interesting thread. Let me preface my comments by saying I have owned and operated a mid sized painting company for 12 years. We have use most of the paint lines available today.
1. 90% of primers sold today are not going to help with coverage. They are designed for adhesion purposes.
2. deeper (not necessarily darker colors) have less solids and more colorant added to achieve the deep color. This is why it takes multiple coats to achieve the designed shade. It has very little to do with the existing color.
3. coverage and adhesion success also depends on the finish you are using (flat, eggshell, semi-gloss) and more importantly the finish you are covering. You can buy the most expensive product from any of the big three paint companies (Ben Moore, Sherwin, Glidden/ICI) and you will have trouble covering semi over semi (cabinets for instance).
4. As important as the product you choose are the tools you use. Countless time I have gone on an estimate for an unfinished diy project and notice cheap dried up rollers and brushes that are a huge factor in the clients frustration. A quality set of tools are pricey but undoubtedly washable and can last you many many years.
In closing, we have worked with all types of paint from all sorts of companies and they can be compared with the big sneaker companies. Nike, Reebok, Asics, etc. They all make a 20$ pair and 120$ pair. As most things go in life you get what you pay for.