One thing that never ceases to amaze me is the exceptions to the rules on DSL. I've seen solid 1.5Mbps connections on loops as long as 22,000 feet, and crappy connections at the same on under 10,000 feet. Even when all available techniques for getting the most out of a circuit are employed, sometimes it simply boils down to the type of cable the signal runs through and what other circuits occupy adjoining pairs.
In that vein, it's reasonable to try to get more speed out of circuits that don't meet the rule of thumb, but often counterproductive to touch those that do. I.e., if it ain't broke, don't 'fix' it!
It's also worth noting that telcos use a mix of technologies to deliver DSL. Just because they serve one area with G.DMT, doesn't mean somebody next door isn't running on the original CAP! (Yes, there are still a few stragglers around here.) It's also worth noting that proximity to the central office isn't the end-all indicator of speed potential for non-fiber DSL. I live less than 1000' from the central office that serves me and can only get 12Mbps/896. People further out running on fiber fed remote DSLAMs by contrast can get 40M/20M! (Knowing the system, it boils down to a trunking layout issue, but that's beside the point.)