Iron out should be safe on corundum.  I do stay away from the stronger acids.  I also watch out for the fumes that you can get even with the iron out.

There's probably nothing more you can do to them.  You can't turn a non-gemmy piece of massive corundum into a gem no matter what you do.

Yeah I'm pretty confident that the grey color is the true color of the rock after all stains have been removed by the Iron Out especially if the rock is corundum or quartz.  Iron Out would never change the color of corundum or quartz by itself.  If you left the specimens in the solution so long that the iron dissolved into the solution re-precipitated on your specimens then you would be left with an ugly brown stain that is very difficult to remove, but it's not grey.

Yeah, I also have some grey corundum in my collection too.

Rocknerd, I just soaked 3 pieces of corundum for about 36 hours, they look good today.  I'm not sure what is going on with your ironout situation. I soaked some very stained quartz with them and they are half way to being clean with no gray whatsoever.

Rocknerd, with Ironout I may use warm tap water, but mostly I use cold water.  It takes more time to get the results I want but I have more control over the finished product.