I personally have top mount levers on all three bikes (two race, one commute/rain bike). I think they are a big advantage on technical courses, especially really bumpy sections where you can ride "lighter" on the tops & carry more momentum into corners/barriers before you have to scrub some speed.
However, my team director has a good argument for not running them. If you get used to always having them there, you will most definately use them too much, ie. brake too much! Totally depends on the course though. At Star Cross I dont think i touched them more than once/twice per lap since you are powering out of grassy corners every few seconds & you get more power on the hoods or drops. To each his own though.
I agree with Blast 100% on the commuting thing, they are very useful.
No solutions for the K-Wing bar situation. I run the RD 200 bars & they are lighter & you can use top mounts.
DP