I can't talk about TAs specifically so you're kind of putting me in a bind, but the total cost difference between a GTY and picking our own is $300 CAD.
Through the Princess website, an Interior Cabin works out to $3,435 CAD (before tips).
Through the TA, a GTY Interior is $2,819 CAD (before tips), and choosing our own is $3,069 CAD. So yes, it's still a good deal to choose our own (compared to the standard Princess rate), but it's a difference of $250.
Also, when I mentioned "losing the ability to get upgraded," I wasn't talking about bidding on an upgrade. There are situations, like the first person who responded to me, where you book a guaranteed inside cabin but get bumped to oceanview or balcony without needing to bid on an upgrade.
Anyways, while I appreciate your help, I wasn't really looking for tips on which rooms to book or whether or not I should book a GTY, I was looking for information from people who have booked them and what their experience was - did they get bumped up a category, did they get stuck in the worst room in the ship, etc.