It's more than likely the turbo, What is it, how old is it? however a compression test probably wouldn't go amiss, In terms of still driving it I'd say no, at least not on boost, I don't know what's gone on with your car, it could have just started smoking on boost or been doing it for a while, if it's been doing it for a while don't risk it. get it checked out, it could be something small and you'll be fine or it could be the start of a much more serious problem. It pays (the garage) to check early and nip it in the bud.
welcome to the expensive world of R ownership :doh: