15lbs sounds about normal. The question is whether the compression is still good and whether the turbos are actually putting out that much boost. More than likely a turbo rebuild will be in order soon. Of course, you should always do due deligence on ANY used car you intend to buy...particularly a sports car and ESPECIALLY a twin turbo car. Chances are grandma put the pedal to the metal on a regular basis as everyone at some point with a turboed car becomes a boost junkie.
'03 Nissan 350Z Touring 6 Aero Super Black Stage Stock '97 Nissan 240SX S14 SR-Turbo Obsidian Black Stage X '90 Nissan 300ZX Twin Turbo Aztec Red Stage XIII