well... let me ask you one thing: when Opeth were just an unknown band playing raw death metal in a cellar in Sweden, and they decided to insert proggish things in their music, weren't they experimenting already? and when Mikael started singing clean, wasn't it an experiment? I say that experimentation brought us that incredible sonic creature that is Opeth, so... what's your point? They have recorded five masterpieces on a row... and I believe they are able to write another two!!!!
on another note, I think the mellow album should be regarded as the sixth, so if it comes out to be a bad disc, we can always say that with the seventh (the heavy one) they have come back to the roots!!!