Lots of interesting discussion here. I'd like to address some points:
Test show that while dispersed do get an early lead that lead is not all that big and concentrated just dominate when lines are at 100% efficiency. Every test I know of show that dispered while holding a lead for around a year to a year and a half (maybe even 2 years) never have much of a lead in terms of raw quantity and then both lines hit peak efficiency concentrated just role away in terms of raw quantity.
And raw quantity is what production lines are about.
This is basically how I've run the math in my games. Not all countries benefit from concentrated in all situations, but if you plan your game around it (and you will not be bombed a lot), it reaps dividends.
Given the time it takes to reach 100% efficiency, it only actually makes sense to take concentrated when you're going to be building mostly the same stuff for the entire game,
This is really important. For the reason Denkt posts below:
One strategy is to jump over tech levels in favor of other research and in such case you may be stuck with low tech stuff for a while. Like you ignore infantry weapon research between 1936-1941 or so and then just tech straight to equipment 3. It depend on you country's situation, like you may need infantry but they don't need to be well equiped in such case it may make sense to keep producing low tech weapon but save your research on more important stuff.
It works like this with tech NFs, too.
So, let's say that I'm going to spam tech and NFs to get 1944 fighters and bombers as the UK (by 1940). You could just run those production line with crappy 1936 planes even when you research the 1940 fighter. Then you jump to 1944 fighters with Supermarine, dump 500 XP into them before you even start production, then swap your lines over.
Then you just ignore them for the rest of the war. The only time they aren't at full efficiency is when you raise your industry tech.
You do the same thing with strategic bombers.
For other countries, I often to the same thing with ART II. Once I get ART II, I might delay (or even never research) ART III. Those production lines stay running for years and years.
Support equipment, while not that important, is an obvious candidate for benefiting from concentrated industry.
German tanks are a different animal entirely. Dispersed might make a lot of sense for Germany, even if you don't care about strategic bombing (which you should care about in many cases).
I've also used the same technique with CAS. I'll have the same X number of factories on the same model of CAS for maybe four to five years. 1936 CAS, while not great, can be produced in great enough numbers to make a huge difference. You keep that 1936 CAS line running for years, and you'll have plenty of CAS. (I find numbers make a huge difference with CAS.)
Denkt points out infantry equipment, but I'd like to add that, due to losses, there are times you may even want to keep producing Infantry Kit I when you research higher techs to make good on equipment losses before swapping over.
To be honest, over the past year, I've gotten more and more convinced that there is real value in just producing "good enough" equipment in 90% of categories, and just keeping the production lines going full throttle for years on end, rather than trying to be cutting edge. You may pick one area to be cutting edge in your tech, but in most categories, stick with tried and true equipment and spam its production.
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