Matt,
I don't think St. George's is at the top of list of Thompson designs that remain purely intact. As you know, holes 3, 4, 9 and 15 are Robinson's. And, I'm pretty sure most of the green surfaces have been altered.
There have been some significant changes made at Capilano, too, including new greens at holes 6 and 14.
Although the bunkers at Highlands Links are screwed up, big time, Thompson's routing and green surfaces are left intact. Jasper's pretty close to original, too, I think.
Thompson did a few courses in small towns throughout Ontario. I'm sure a couple of those layouts haven't been touched.