I Probly only raised exhaust 2 degrees, mostly to smooth out the port and sleeve transition. I took a good amount out of lowers and definitely the secondary’s to improve entry.
Didn’t buy any of it as a kit, just been adding to it over the years.
Maybe you could try soaking the inside with CLR? Then pressure washer.
If that doesn’t work you could try a blast cabinet. I’d be a little Leary of that though, because you’d have to make sure to get all the media back out of the pipes.