You are likely to get sync rates just shy of 40,000 and 10,000 kbit/s in cases where your line is capable of more, but the observed throughput
rates will be lower for a number of reasons. Downstream there is a "profile" in place which acts as a rate limiter. There are overheads for every protocol in the stack that your data is wrapped in, even if there are no errors, and there are retransmissions (due to transmission errors caused by line noise) to account for. For ADSL it used to be that you should expect 85% of your sync rate as your maximum throughput. For FTTC downstream I think this is more like 92% (hence getting a touch more than 37Mbit out of a 40mbit sync rate) - this agrees with what I saw downstream on 40/10, though upstream I never saw more than ~8.5mbit as an observed rate.
Note that because of the complications of analogue comms, as used between you and the cab, you are not guaranteed to get a 40/10 sync rate on a 40/10 product if your line would give you more on 80/20. IIRC the 80/20 product uses frequency bands spread over an 18MHz range were the slower product just uses a subset of those (over a range of about 9Mhz) - if your line experiences a good bit of signal loss in that subset of frequencies then you are going to get less than 40/10 on the 40/10 products but might
still get more than 40/10 on 80/20 products (but not the full 80/20) because the extra usable bandwidth in the other frequencies makes up for the deficiency in the other bands.
Current Line: Andrews & Arnold (AAISP) via 80/20 FTTC, getting close to the full rate both ways, joined July 2011, upgraded from 40/10 to 80/20 May 2012.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.