TMP isn't really about lower tolls, although that helps too.
Western Canadian production has been restricted to US markets, so negotiating power is limited and the buyers have been able to take advantage of the situation. Once significant production volumes gain access to different markets via TMP, pricing could end up being determined based on benchmarks other than WTI - e.g. Brent. This competition should also serve to improve pricing for the production that continues going to the US.