About the same tactical surprise that a thug holding a loaded gun with a hair-trigger at your temple could achieve. Locked and loaded, armed and ready, they are that close to downtown Seoul.
We have F-16s at Osan and Kunsan; F-15s and more F-16s in Japan including Okinawa. The Navy and Marines have F-14s, F-18s, and intruders in the area. For a really big punch we have B-52s at Guam.
The air power we have is unparalleled, but to counter this the North has placed literaly hundreds of sabotures in the South, their famed 'special forces' brigades are ready to infiltrate deep and quickly, and their missile attacks will target our airbases.
There are no B-52s routinely stationed at Guam, but we may move some there.