I currently have 512kps download speed ADSL service from TOT (at least that is theoretically what I have). According to the TOT rate sheet, for just 20 baht more a month, I can double that speed. And it doesn't cost that much more to increase the speed even further. Does it autoatically follow that subscribing to a higher speed will give me better service? Or does it depend how many people are on each server? Is this something that is difficult to generalize about? In other words, each person's experience is different?

Yes .... go out and spend that 20 baht ...... It's one Kaow Phat less, but it will be worth it. Especially if you don't sleep during the morning hours ... I would say go for the 2Mbs package.

When they upgraded from 1 to 2 Mbs my DL speeds (especially while using a DL manager) doubled.