Neil,
Today I get Pro Satin paper and finally made a print with profile you send me -- and it's OK. And I have found one thing -- soft proof with that profile looks quite well, both intents, perceptual and relative. And the difference between them not so huge.
But with the "easy" profile for Epson Premuim Glossy I see a big color cast, especially in perseptual rendering intent, especially for blues. The prints are the same -- foozle. This makes that paper not that much usable.
So it seems that internal profiling is not working correctly -- may be only for that paper, I don't know.