All 08+ Imprezas other than the STI have open front and rear diffs.
OP with VDC/TC off, there are no electric nannys left other than ABS. On the STI, there are 2 levels of turning it off, but on the WRX, it's on or off. You DO want it off for autox/track as it is terribly castrating. With VDC on, it not only brakes individual corners as slip is detected, it uses steering angle and lateral G sensor input to cut throttle too.
Don't get too caught up in the VLSD vs open. VLSDs lock so slowly (speed sensitive) that they really don't help in performance driving unless you are in a humongous steady state sweeper on throttle which is infrequent. They are more for inclement weather driving. A real clutch based LSD would be the way to go in the rear....1.5 way would be good.
However, I think a more night and day decrease in track lap times would be noticed after installing a helical/gear/torsen style front diff. Sure if you get front inside wheel spin, the center VC will xfer torque rearward, but again VCs are slow so keeping traction upfront and exiting the turn on throttle will be much faster.