Vertical Perspective correction, degradation of image.

I am using high quality Nikon, Canon and Zeiss lenses around the 21-24mm focal length to photograph churches, convents and other buildings with high towers etc.

My problem is using photoshop to correct the converging verticals present in these images.

For example, the church cross at the top of the image is always softer than the lower part after straightening the verticals in photoshop. The sharpness of the cross is acceptable being near the edge of the image circle before using perspective correction, but unacceptable thereafter.

I do appreciate that photoshop will degrade the top of the image if I stretch the pixels, it is good, but not good enough because clients are commenting on the slight softening of the image tops.

I have tried Perspective Warp, Lens vertical correction, Free transform, Transform skew, Transform perspective and transform in ACR, but they all seem to produce identical results.

Can you please tell me which photoshop method is the least destructive, if there is one?

I could resort to using Canons 17 and 24mm tilt and shift lenses but thats an expensive outlay which may not produce the answer?

For the moment I am using ACR's adjustment brush and brushing in some extra sharpening where required, not the best solution, but I can't find a better one at present.

Any advice you can offer will be greatly appreciated.

Regards

Michael