I’m not sure I completely understand your question: the faster ones take less time to download things.
If you’re asking if they actually cap the bandwidth at the specified speeds, the answer is, in my experience, yes: you’ll never get more than momentarily faster than the rated speed of your DSL line.
As for practical use, after 256K bits/sec, you won’t notice the extra speed much when web browsing sites that aren’t image heavy. After 512K, you won’t notice it much for any web sites. But when downloading files, you’ll get the maximum of:
- What you can download at.
- What the downloading site can send at.
- What the current conditions on the Internet allow.
I’ve had DSL here that’s slowly increased from 640K to 768K to 1.0M to 1.5M to 6M over about four years (at about the same cost, no less!). Up to about 1.5M, I really noticed the download speed increases. At 6M, it’s more of a wash - that’s about 600KB (bytes, not bits) per second, and only about a third of the sites I download things from can send that fast. During the day, Internet congestion almost always limits me to 200-300KB/sec, at night I can get 600 or more from select sites.