The stock engine can handle a lot of boost on E85 without issue as long as the tune is good and the car is not knocking. That has been our experience with an 05 STI running 25 PSI peak most of the time and sometimes we turn it up to 28 PSI. The engine in that car just ticked over 100k. We don't expect it last forever, but we are looking to see how long it goes.
That said, if you are really determined to do a build, replacement rods might be a good idea. The stresses of high reving on the track over time can fatigue them. How much do OEM replacement rods cost?
Why choose Wossner pistons? I have not seen those used very often for Subarus, unless I missed something?
Phil from Element Tuning: “We raced on the stock engine for an entire year at 500 horsepower just to prove the point that these engines aren’t fragile—with the right engine management system and tuning, as long as it’s not detonating. Other shops were betting that we would blow up. We beat the big guys, and the engine never blew.”