Volts/turn is also a function of frequency as well as u (and magnetic path length). In the 3c9x (x = 2 or 3) stuff I have, I'm using 8v/turn at ~~18khz for a 1" cross section (1x1), it would scale as you go up in F more or less. Low u stuff means that in general you have to wind around the whole core, or lose a ton. I found this out when trying to use some amateur radio balun cores with the primary on one side, the secondary on the other for DC isolation. That one just does not work, far too much field loss - low u implies that it's not good at guiding field around corners.
Even with the high u stuff - I had to make my 3 turn primary go most of the way around the core - putting the 3 turns tight on one side had much too much leakage inductance to the secondary (but a little is good if you want to drive with a sq wave, but don't need a sq wave output) Both Spellman and Glassman use that trick in their special transformers, so their switches don't try to drive what amounts to a short circuit with the fast edge frequency components well above the self-resonant frequency of the transformer. In this case, the leakage inductance is the needed low pass filter between the switch and the transformer.
At any rate, at 500khz, volts per turn isn't going to be much of a limit for most reasonable things.
Trying to get super high turns ratios is not a reasonable thing (and why I've launched some threads in the attempt to attract some real experts who can tell me how far I can push it best case), and is in general limited by self resonance of the many-turn winding. Going to higher u to reduce the turns doesn't really buy you much either, I've found. In fact, this truth is why there are such things as CW voltage multipliers, rather than just winding a super-transformer and using a single rectifier -- even at mains frequencies.
Remember that a transformer has the most "B" in it when unloaded. This is why most of the X ray transformers we've bought can't work for a fusor. They saturate too easily unless they are fully loaded and burn up on even short runs if you don't limit the input current - some heat significantly even when fully loaded in a one second X ray exposure. Drawing current from the secondary cancels some of the primary's field is how that all works, similar to the concept of back emf (but different, that's a gross analogy). So things that have a ??? load (or even open circuit) are much harder to do and much more expensive than things with a well-defined load - see the prices of AR wideband amplifiers and compare to a ham transmitter...about 30::1 or more ratio there.
You might fool with grinding up some ferrite and adding more binder (epoxy) to reduce the u if you want to. I've used a rock tumbler with alumina media to fine-grind things like that, takes a lot of patience but not much sweat. Ferrites in general are good to the mhz region at least. Did you check specs on the link I gave you yet?
Here's some stuff downloaded from there, gheesh.
They also do iron powder, mo-perm powder (the best) and you-name-it. No need to measure this stuff's BH and hysteresis loops; unless that's your main project. Just get something with specs and do the project.
Call them if they don't have what you need. You might be able to social-engineer a couple samples, I've done that. You sometimes have to do a little work to get what you want in that game.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.