Clutch or spark plugs?

Hi, I have a 2.0GT Legacy (BP5) 2006. The power between 2000 and 2500 RPM is restricted - it doesn't lose any power or torque between this range, but there is no further acceleration until it gets over about 2500 and suddenly the acceleration kicks back in again. It happens throughout all of the gears and very rarely outside that rev range. It started happening suddenly two hours into a motorway and A-road drive. My regular mechanic (who doesn't have loads of Subaru experience but is generally reliable) tells me is definitely the clutch. The biting point of the clutch is very high. My friend who is a part-time mechanic and previous Legacy owner tells me it's definitely the spark plugs, they haven't been changed for at least 10K. To complicate matters there is other work that needs doing, so it would be really helpful to understand a bit more before I start spending more money on a car with 192K on the clock. TIA