IMHO, this is bad advice. I seriously doubt you are spinning out in your highest gear you currently have on your Mtb. The bike leg of triathlon is about a sustained steady effort. The only time a mere mortal might need an 11 tooth gog is for sprinting, which doesn't apply to a triathlon If you find yourself going along in your biggest gear spinning 90rpm and saying to yourself this is too easy, I need a bigger gear to really get myself tired, then 1) buy a 11 tooth gog, and 2 consider doing this for a living because you are genetically gifted.
Gears do not make people fast. Its the engine on the bike. learn to spin a good cadence in the 90-100rpm range and you won't be looking for the biggest cog. Buying new gears is money that could be save toward a nice new road bike.
Also I think its a waist of money to change the mddlie front ring. If its not big enough shift to the outer one. In my experience in tt's and tri's , your not shifting gears much. Its more of a matter of finding the right gear that allows you to maintain the right cadence and effort and then spinning that gear.