1. The checkers are estimates, so for one person they can be spot on, and another they can be way off. All the xDSL providers take their core data from the same source, so until you get the line running it is very much just a ballpark number
2. If you mean can you watch a HD stream and it not affect lag, then you will not get that from any xDSL provider. You need a connection that is several times faster, i.e. to the point that a HD stream is a small part of the link
Downloads are the same, ie. gamers generally do not share connections well.
Webpage loads are a lot to do with the DNS lookup time, and you can change this yourself. If you are looking at graphic heavy pages, e.g. ones with 1280 x 720 images on them, then you'll notice a slow down (unless VM was performing badly) but normal websites, the difference on a 5Meg versus 20Meg is minimal due to the way a webpage is delivered.