I have investigated this further and it appears that the information T-Mobile have given you below is correct, although contrary to the information I have been given previously by T-Mobile.
At the time of ordering, the T-Mobile 'Web and Walk Plus' booster was only available to customers not purchasing directly from T-Mobile at an extra cost. This addition not only has a higher data allowance under their fair use policy but also gives increased data speeds above those of 3G where coverage allows. Customers purchasing Android handsets directly from T-Mobile received this at no extra cost. This has now changed and T-Mobile have brought their offerings regarding data inline with what is available through independent retailers.
I have discussed this with senior management here and we have only ever specified the quantity of data considered as fair use in our advertising and not the data rate so there is nothing we can do. If you feel that T-Mobile have misled you through their publicly accessible information/advertising then I would suggest submitting a complaint to the relevant ombudsman as it appears further dialogue with them directly may be pointless.
I'm sorry there is nothing more I can do to help but I do hope now the confusion surrounding this issue has been removed.
Customer Technical Support Manager