One has to be a bit more specific about "historically", and again about the scope and level of the interaction.
CP may have had bigger asks at times, and yes there have been difficult discussions. But also periods of cooperation (I'm old enough to remember those weekend GO locomotive leases, and the hoops CP jumped through to return the locos on Sunday night as promised)
This is just it.
The quid-pro-quo for this was seemingly that GO was simply allowed to operate on the Galt Sub at all - and the language in the memos seem to reinforce that. It wasn't until the F59s arrived that GO was able to pull out of it, as they claimed that they were specifically designed as passenger locos and really would not do well in freight service (which is quite the load of horse manure but seemed to make the case for them nonetheless).
As for the hoops that CP jumped through, there were a certainly number of occasions that they did not return the units in time, or fueled up, or properly serviced. In the 1970s and early 1980s Mondays had the worst OTP as trains had to be cancelled, or set swapped out in the middle of the day lest they run out of fuel in the middle of nowhere. At that time it certainly seemed that CP operated on the basis that GO was lucky to have them as a host and should be grateful.
That said.....things did seem to change in the early 2000s. CP approached GO for the use of their trains when they brought their steam loco out east in 2002 and 2003. They've been more amenable since, although ultimately their bottom line is still their ulterior motive. They still remain fairly inflexible regarding their scheduling on the Galt Sub, for instance.
I'm sure they were basically cooperative about the West Toronto and Davenport grade separations, for instance. (For obvious reasons, the projects were helpful to CP's operations) They have participated in numerous studies (including the last round of Milton studies, a decade or so ago). The sale of lands at Obico didn't reach the newspapers, so I assume it wasn't completely acrimonious. They have not said "no" to HxR, and they have interests throughout that project in all its flavours.
In the case of West Toronto, they absolutely were cooperative for that reason. Meanwhile in the the original Bolton reports of the late 1990s, they were quite adamant to limit GO's abillty to operate any trains beyond the limited slots they were willing to allow.
They've certainly pushed their own agenda in other projects, too. Look at the Hamilton Tunnel - the expansion of the tunnel wasn't done for GO's sake, as BiLevels fit into the tunnel already. And yet CP made sure that GO paid for its rebuilding to allow bigger freight rolling stock to suit their own purposes.
There are undoubtedly precedents in the Bowmanville deal that CPKC knows will bite them when Bolton GO, and Milton 2WAD, come along. So I would expect they would be extremely cautious and ensure they were not setting themselves up for a future problem. Bowmanville is not a "one and done" situation.
I don't think that's true at all. There's a reason why GO is going to have its own track to Oshawa, where the regular all-day service will end. A small handful of one-way trains running out to Bowmanville past Oshawa won't affect their operations in any meaningful manner, and they know that.
And then there's the whole issue surrounding TC and the RAC, and their relationships with all of the various railways. Neither GO nor VIA (or WCE and EXO) want anything to do with RAC, and TC is certainly doing them no favours.
CN's involvement and positioning in the defunct Bypass discussions or the Halton line upgrading would be an interesting comparison. Of course, even there, they had an intermodal yard about to be built... so how many feathers did they choose to ruffle? And CN seems to play a larger role in lobbying eg via RAC. (Let's be glad that GO never bought Ventures).
There may have been objections in the boardroom, but I have talked to enough ex-CP RTE's who worked the Milton trains to believe that in the day there was a lot of commitment and pride to running those trains well.
From the running trades, I have no doubt.
From the guys above? They had no interest.
And in my own career, my corporate real estate colleagues who worked with CP regularly (we adjoined their property in plenty of places) described their CP connections as a bunch of particularly difficult SOB's.
And I've heard similar in other industries as well.
Dan