the way i see it if a Subaru dealer is willing to certify it and SOA is willing to hold up the remaining 3 years & 60k miles then Its the safest you can get without ordering a new one straight from the plant.
I have seen some old ladys driving WRXs like Sunday cars in the past. If the interior is clean and the exterior is clean and there aren't any signs of young driver mods ( HIDs, LEDs, Excessive stereos, Black-Out Tint) then I would get in on that, assuming the price is right.
Ive purchased previously leased cars in the past, they have been good cars. pricing is usually a bit below KBB but i think that's just a sign of the dealer trying to recover the reaming value after the lease. keep in mind that most lease contracts also include scheduled maintenance whick means that at least for the time is was leased that the car was maintained by Subaru.
If it were my money ( you didnt mention how much they want for it) i would go for it : )
Slow is Smooth > Smooth is Fast