I used a commercial heavy duty floor cleaner/degreaser (called Lift100 or something, left over from a previous life) and applied it direct with a toothbrush, agitated it, then started dipping the toothbrush in a jar of water and scrubbed it some more, finally sprayed it off with clean water from a hand spray (repurposed household cleaner bottle) and wiped dry. Mine came up OK. Admittedly they actually weren't to bad to start with. I was tempted to use WD40 or paraffin which I reckon would shift the muck but you'd have to make sure it was all removed.
personally i would just hand em over to the powder coater and get em refinished in black for about 30 quid a calliper. then keep on top of the cleaning.
You might try gunk, and use a dremmel with nylon brush attachment (eBay) to get the crap off once its soaked in for ten minutes or so. Shouldn't be a big problem, mine weren't!
I remember freely using trichloroethylene to degrease components when at work. Others used it while smoking..
If it's baked on brake dust you are referring to.....then Bilt Hamber Auto Wheels....may need a few hits but there's no wheel I've had which has not been completely decontaminated with this stuff....it will turn purple indicated it is dissolving the ferrous metal particles...
Every time Rob recommends a product I google it. Just sat here laughing as pictured a “run” on Bilt cleaner, a bit like hand wash.
Now that hand wash supplies are exhausted I am resorting to motorcycle cleaning products. I might leave the Harpic until the zombie apocalypse though.