[pile o' wrx]

Factoring in the mass of the clutch/flywheel as a possible subjective cause of your drag?
Do a battery hygrometer test, check the cables for corrosion, and do an amp check before ruling out a going south starter.

STI's come with the standard drive type starter, whereas earlier generations of Subies came with gear reduction starters. I prefer the earlier style units for ease of starting ability.

Believe the starters are the same whether 4eat or 5/6mt, not interchangeable. Curious as to whether the earlier gear reduction starters mount in the standard drive location. I've never rebuilt a standard drive unit, but the parts to rebuild a gear reduction version are about $8, my cost.

--Correction-- Different part numbers for both the 4eat and 5mt versions.