All tyres shed rubber, there is always a clean and a dirty line. It makes no difference is there's more 'marbles' off-line (or even none at all), it will always offer less grip (off-line) because it is off-line.
Pirelli said that they'd make tyres that would have a certain lifespan in race conditions. They've said the same in the past and have done exactly that. We've yet to see how they perform in race conditions. So far we've only seen winter testing. They say that they've produced a tyre that is more easily "understood". I see no reason to disbelieve them but like last season, the variation in chassis performance will mean that some teams will find it easier than others.
It's fun to speculate, it's interesting trying to work out which teams are struggling and which are sandbagging but we won't know the truth until the first few races are done and dusted (last year, the first race performance of McLaren flattered to deceive, it looked as though they were going to dominate - Ha! How wrong that turned out).
Archibald Tarquin Throttle-Whistle Esq.