There will be some theoretical reduction in capacity attributable to a "flat" junction as opposed to one with flyover/unders, but ML seems to feel that it's manageable. It may mean that more trains face restrictive signals as there will inevitably be more routing conflicts. But it may be years before the line gets that busy, and changes to the signalling will likely help. Many trains will only be moving at turnout speed anyways eg if they have stops at Scarborough GO, or if they are coming around the curves to/from the Uxbridge Sub, so routing them through crossovers does not impose a speed penalty.
Wice on the Kitchener line is a comparable junction, and it functions reasonably well.
- Paul