Not sure how KB->KD IO’s are any more of a “Mastermind thing” than a Storm thing, a Kheldian thing, an Energy thing, a Bonfire thing, etc. I happily deal with it on all of the above because the KB->KD IO’s are plainly worth it except on S/T attacks.
On some powers like Tornado and Bonfire, there is no other enhancement as potent as a Sudden Acceleration/Overwhelming Force in DPS terms. So obviously the knockdown conversion should retain its cost, because powerful things have costs in game design and balance.