CP are tough negotiaiors, nothing more. They will hold out for the best possible deal but once they have enough money on the table they will find room for agreement. I'm sure they recognise how they could upgrade at GO's expense.
One interesting thing to note - there is longstanding legal precedent that a railway diamond is controlled by the railway that was there first, and costs are borne by the railroad that came last. In the case of the Davenport diamond, the GO line was there first, in fact it's Ontario's oldest rail line. If Metrolinx knew how to be as tough a bunch of SOB's as CP is, they would have a lot of leverage to negotiate the solution.