I went ahead and did this mod yesterday, I'd post pictures, but it's pretty much identical to what the OP did. To answer my question, the machine has no problem cutting mild steel down to 600 rpm, the VFD does a great job of maintaining the rpm. As an added bonus, my cuts are looking much cleaner than when it was gear driven. I'd imagine that's because there's no backlash now between the motor and the spindle, where the gears would obviously have a bit of play.
I'd recommend this project!
Thanks for taking the time to post your work, you've saved me a bunch of time.