Sync speeds and actual speeds

[The Dong]

I have a 2Mb fixed speed just now on ADSL.
My modem reports a sync speed of consistently ~6Mb+

If I regrade my ADSL service from fixed to 'up to' 8Mb ADSL, will I 
actually get the speeds my modem can sync at? Remember, I'm in the rip 
off UK ;)

Reason I'm dubious about getting what my line is capable of is because 
the online checkers (BT) say my line is only capable of 2Mb/s or more.
ADSL MAX is available.
Other reason is that all the surrounding routers I've 'probed' are 
getting very poor sync rates of about 3.2Mb/s.

What I'm really asking is if BT and other unscrupulous doodoos are 
artificially throttling sync rates across the board; will I get anywhere 
near 5-6Mb/s, or am I delusional to suppose that my optimal, current 
sync rate would not 'up' to a real rate?

You see. If I 'regrade' to the up to 8Mb service, I doubt I can go back 
if it goes tits up and by some small print near omission ruins my 
current unlimited use service..

I'm still dubious ;)