Just a small point, it isn't really a backbone as such, the providers here "peer" to each other, and some providers have peers in other countries, so it's not a simple path the a "backbone" as such.

But your point is essentially correct - your access speed is meaningless if the provider doesn't have enough upstream bandwidth. Providers tend to keep very quiet about their contention ratios, but I must say that in the 4-5 years since I've had broadband the providers seem to be quite well behaved in this regard, adding bandwidth and additional peering as required.

Another aspect to consider is not just your bandwidth and that of the larger "pipes" (for example to the US) but that of the servers and companies you connect to. Often if downloading a large file from somewhere it is not likely that you'll get your full 600Kbyte/sec for the simple reason that the other end's upstream connection just isn't that fast! After all if you send a file to your friend and you both have 5000/500, the effective download speed for him will only be 500!