XP-1 was processable in C41 chemistry but it wasn't fully compatible because for optimum results it needed a longer development time, most small labs couldn't handle that.

I've no idea whether the Ilford XP-1 developer was more dilute, I don't think so but it was expensive compared to Photocolor II which I already used, I still have the original datasheets back in the UK, I was reading them about 3 weeks ago.