Yes, you can mount the fan below the heatsink fins and then it's only 115mm. Very capable cooler for a low profile model.
You wouldn't want to replace the fan with another model unless perhaps it was the NF-A14 chromax.black.swap or chromax.black.swap industrial iPPC 2000rpm models, because the NF-A14 that it comes with is a highly superior fan with excellent static pressure, sound pressure level and CFM characteristics. You won't find another fan AS good as that fan that moves as much air as that one does, while having excellent high static pressure AND nearly silent operation. Believe me, won't happen.
The closest thing in a slim design would probably be the Cryorig XT140, and it's not nearly as good especially in terms of static pressure and noise level.
http://www.cryorig.com/xt140_us.php
Might be ok as an addition though, to use WITH the NF-A14 since the Noctua fan will tend to have a higher overall CFM so it won't be a restriction to the XT140. Honestly though, I'd probably just the Noctua by itself. I do expect Noctua to be releasing a slim 140mm version of the NF-A14 or A15 at some point, much like the NF-A12x15 120mm slim fan they released recently. No telling when though. Noctua tends to take a long time developing products to ensure they are without equal, but since they already released the 120mm slim model that MIGHT indicate that the 140mm model has already been in development and could come sometime this year, maybe.