The issue isn't whether I think 5 gigs is reasonable but whether TMobile materially changed the meaning of the word throttling after he signed the contract. TMobile can (of course) have an engineer study the logs on his account and argue that illegal tethering was going on and the poster was in violation of the contract.

TMobile's terms and conditions have an arbitration agreement with the American Arbitration Association (Consumer Arbitration). Their website is adr.org.

You need to start your dispute (after finishing with customer support) with a letter to TMobile's legal counsel outlining your dispute and requesting resolution. The address is in the contract. After you are done, you can file a petition for arbitration with the American Arbitration Association. Read the contract three or our times.

If a contract term is not defined at the time of signing, an arbitrator can look at practice to define the term and you can argue that excessive throttling is a violation of the term. Next, you need to think what your remedy is. It is much harder to get specific performance than recession. It is much easier to get specific performance of something where TMobile doesn't have to create a specific plan for you. There is a balancing here. Your detrimental reliance damages go up if TMobile made you lose opportunity.

For example, I have an ATT unlimited international data plan which I am grandfathered in on. If ATT tries to argue that I am not damages based on the cancelling because they are no longer billing me the extra $35 a month for it, I would argue that had they spoken up earlier, I would have gotten Verizon's competing plan, specific performance is the only way to make me whole, and that ATT is still running precisely the same plan for others. They won't have to write a special plan just for me.

Aribtration is a pain, but you can win. Take your time, make your arguments. If you have a friend who is a lawyer, bribe him/her to proof your stuff.