Sneaking success is affected by armor weight, so I would guess that even if sneaking in power armor is technically possible, it will be very ineffective. Which makes sense, of course--an inability to be stealthy is probably the most common-sense drawback to a suit like that.
hmm... you even can install a stealth-mod for sneaking^^ so i think sneaking in PA wouldn't be so bad at all^^
would you mind *spoiler* me where you can buy the cores?