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Duplicate Layer (BG copy)
Transform>Skew @ BG copy)
Duplicate BG Copy (BG copy 2)
Sharpen BG copy 2 to offset the pixel manipulation from the skew
Reduce Opacity of BG copy 2 to taste (usually around 85%)
On this occassion, I pulled the right corner to the right for the majority, followed by a little tug to the left upper corner. As you can see, the pine tree hasn't been changed much from the original, but the middle and right sides are have been corrected a bit more.
The outcome @ losing some real estate is still a bit of an issue, but using Skew allows you to decide which side of the image is more heavy handed than the other vs. applying equal amount to each side by default.
A subsequent tug of the left corner up & left brings the pine tree a little taller/straighter if wanted.
But, no matter how you go about it, correcting distortion eats real estate. So, for my money, it is better to shoot with a well corrected 18 than a distortion inducing 17 ... or something like that. Also, another reason why I like the TS-E's ... their larger image circle (while slightly contrast reducing) provides for better corrected projections as you aren't using (in normal mode) the edges of the projection where distortion is most prominent, i.e. a built in crop from the projection's distortions when the plane isn't kept vertical. Of course, if you are using a TS-E, you've got some other tools at your disposal.