280KHz sounds fine. The only real limiter on resonant frequency is materials available, which for solid state coils also includes calculating the switching time of the transistors involved, and even that can be pushed to extremes.
Btw, if I can figure out how to do the PCB layout I'll be doing a run of my very own universal SSTC driver design. My driver is self-tuning from 50KHz to over 4MHz, self-starting, and will drive pretty beefy FETs/IGBTs. Minimal parts count and is as simple as populate, solder, attach bridge, and go. No fiddling what-so-ever. I'll probably use ExpressPCB because I have a lot of experience with ExpressSCH, but I've never used ExpressPCB itself (service or software). Only part I am iffy on is doing a second layer. I'm sure I can manage a single layer board but I would need to make sure the vias all work and the ground planes can be formed right.
If anyone has any tips or info on the CAD aspect of PCB design I would appreciate it greatly. Hell, if you design the PCB for me you get a board free shipped to you.