Shaving about 3/4 to 1 inch off the height and using a 3 inch bbl. would reduce my FNP 40 to a height to about 4 3/4 to 4 1/2 inches and the OAL to 6 3/8 inches. That to me would make it a sub-compact. A large sub-compact ( oxymoron ) at least. A pocket pistol it wouldn't be, but it would be close over all to a Glock 27.
I think it would be possible to do. Is there something I am missing?
The Glock 27 doesn't carry as many rounds as you are hoping to squeeze into a subcompact FNP. An Hk USPc does carry 12 rounds of 40 cal. In order to do that (what you are proposing), you;d have to have a grip as long as a USP compact.
I am setting here looking at my FNP 40's magazine. It holds 14 rounds, the second hole, indicating capacity, is about 1 inch above the other. That one says 10 rounds. It appears to me that if you shortened the mag one inch, it would allow the grip to be inch shorter, resulting in the above post's overall height. I am no pistol smith so I could be way off, I suppose.
The G27 holds 9 rounds of .40, it seems that squeezing one more in wouldn't be too hard. Then again I don't own a Glock and their engineers must have had a good reason to not make it so.