I think what you were seeing was more due to ATM overhead than the DSLAM trying to be cute with throttling. Because ADSL encapsulates everything in ATM even small IP / Ethernet frames get broken up into lots of ATM cells which can add upwards of 20% overhead. So an ADSL line trained at 8Mb/s will never provide 8Mb/s of usable throughput to the end user. Some ISPs actually advertise targeted throughput instead of train rate and set the train rate a certain percentage above the target throughput to compensate. Others just advertise train rates and have disclaimers in the fine print.
I've had my hands inside most Gen 1, 1.5 and 2nd Gen DSLAMs and never seen any with automatic throttling like you described.
(Gen 1 being units that just function at the ATM layer requiring an external system to bridge to Ethernet or IP. Gen 1.5's being upgraded Gen 1s with crude bridging, and Gen 2 being units that were designed to terminate connections directly from the ground up.)