You bring up many great points...
...But these posts were made long before the release of the HSR studies this month (March 2014 studies released/leaked early December 2014)
https://s3.amazonaws.com/s3.documen...ronto-london-high-speed-rail-2014-03-11-2.pdf
https://s3.amazonaws.com/s3.documen...350/memo-tkl-hsr-impacts-smaller-redacted.pdf
The HSR studies say 40 minutes in the existing corridor from Kitchener to Toronto appears possible with
existing turning radiuses of the existing Kitchener corridor, with no new right of ways (just widening of existing). This is by a firm that has experience doing HSR studies of HSR routes that actually gotten built, so they know what they are talking about (even with a rushed study). This would be new information beyond what you already wrote. The study shows that the new HSR corridors only occurs between London-Kitchener; the existing railroad corridor is used for 40 minute Kitchener-Union. Which makes it an extremely low lying apple for future GO RER Kitchener expansion. Mind you, I'm ignoring the cost of the high-priced and harder-to-initially-justify London leg, focussing purely on the impressive congruence between GO RER electricification and the Kitchener-Union HSR leg.
This reinforces my view, that when Kitchener GO RER needs to intall new extra express passing track, they might as well make one of them HSR compatible, or protect towards a future HSR upgrade (e.g. replacement of wood ties with concrete ties, catenary tolerance tightening, etc). Obviously, I imagine some realigning may be needed (e.g. moving track sideways a few feet), which is something they do all the time worldwide overnight in sections at a time without stopping daily service, and readjusting positioning of platforms at stations (much bigger project) but the corridor is already apparently 40-minute HSR compatible on the existing Kitchener-Toronto right of way (no new ROW needed except widening and track realignments) according to professional mathematical calculations by actual HSR study professionals. If you're realigning anyway for additional new GO RER express tracks (adding track, adding catenary, etc), on an already-HSR-viable corridor, then you might as well protect for future HSR upgrade...
You're right, it may be a costly upgrade (e.g. replacing track, sideways shifting of track, concrete ties, new catenary) if they did not HSR-future-proof a GO RER express track addition (a multi-billon-dollar project, by itself, with relatively little further increment to make it HSR compatible if you don't need new ROW)...
According to these HSR study professionals, the Kitchener-Union corridor doesn't need any turn radius changes to accomodate 40-minute HSR Kitchener-Toronto. 40 minutes isn't the same thing as 1h15 minutes -- it makes it daily-commutable -- and in some countries, makes a rail almost an order of magnitude more popular than it otherwise would be (e.g. just look at France's first HSR that got so unexpectedly popular that route now uses bi-level highspeed trains with tight 3-minute headways, after incremental upgrades that quickly followed afterwards).
This doesn't cover the problem of noise abatement and other aspects, mind you...