Second: ADSL and SDSL in essence have nothing to do with overbooking or internet. It's what the industry has made of it. They wanted to produce a standard el cheapo product voor the masses. Based on the widely available twisted pair, they made a standard, home-to-internet (non-point-to-point) xDSL flavor with overbooking.
Third: ADSL and SDSL come in flavors. From 1/500 overbooked to 1/1 overbooking. From a 'we will fix it within 5 workdays or will try to' to 'fixed within 2 hours' SLA's. Just show 'm the money.
Fourth: DSL comes with guaranteed bandwith if you have the right provider that can service you with the right QoS. Check the ATM QoS for DSL: UBR and UBR+ suck, VBR-NRT is better, while VBR-RT is very nice and CBR is the best available. I mean: on a 1/4 1Mb SDSL connection with VBR-RT quality, you have 1/4th guaranteed, no matter how busy the hinterland is. That's in the QoS standard.
So, in short, a 2.3Mb SDSL connection with Gold (24x7, 2 hours) fixing guarantee and CBR quality is basically a good old T1 connection. And yes: it will cost you, dearly