@ShadowJK are you talking about the IP Profile?
I don't know how it works for business (although I would assume it is similar) but how much you pay isn't directly related to your speeds on BT's ADSL. It is sort of indirectly related though, as the people on the 10GB package can't get ADSL2+, but frankly, you probably don't want it if you only have 10GB/month.
The IP profile is not set by how much you pay. It's set by your sync speed. You are right,you do lose roughly a meg from your sync speed, and then another 1/2 meg for overheads. Which is ironic... because BT say the profile is needed for overheads.
It is impossible to actually get 8Mb/s on a MaxDSL connection, as the max profile is 7.15, which caps out at about 6.5Mb/s actual download speed. On 21CN I believe the max sync speed is 23999 or something stupid like that, with 20Mb/s being the top profile, and actual thoroughputs of 19.2 (that's what I get anyway...)