@shadowlin Hey, I'm not much experienced but I think I could answer some of your questions. When you mine at a pool, the chances for you to find a block is more or less the same of you mining alone, the difference is that everybody mines like they are a single person, so the plots of everyone kind of add together and the chances of finding a block boosts a lot.
About the plots, in the beginning I had 3 plots of 1TB each, and made 15s to read them all (not so good CPU ):), after changing some things, I ended up with 9 plots, and the speed is the same. I think if you make 10 plots of 100GB per TB, you could see a difference in speed, but with that much I don't think so.
The speed of reading don't affect directly in mining, the thing is, if you are in a pool and your plots take 60s to read, the pool may find another block while you are still reading your plots, so the reading stop and begin from 0 for the new block, making you lose the remaining chances that you had from the total plots.
A reasonable time I think would be like 30s most.
Hope this could be of some help :)