The only thing I noticed about my grill delete is my IAT's dropped a couple degrees. I highly doubt that transfers to any measurable HP. You'll have to pull the front bumper off and pull the grills out to cut the honey comb out.
I know of a local guy that has the F14's on his S550 and they look really good. There are some S550's deep into the 8's on 17's so 15's really are not necessary.
If you haven't switched the tranny fluid yet, I highly recommend it. I went with BG Syncro Shift II and it was night and day. Even with the MGW the difference was huge.
I just installed mine using this thread and everything went pretty smooth. I had to tweak the settings a little in the software to get it to work. Works perfect and that 2 step sounds so crazy.
If anything I think it would be slightly easier cause the no-lift feature prevents you from over revving, buuuutttt the RXT is tougher on the tranny whether you have the WOT box or not. I curious about this too, I just installed my WOT Box a couple days ago.