I am running a stock STI flywheel with a stage one HD Exedy STI clutch in my 2007 Spec B, which is my daily driver. I personally like it a lot, as there is great pedal feel and it is super easy to modulate. The clutch is somewhat heavy feeling, so you have to decide whether that is something you find desirable (many people don't like the heavy feel).
A few observations after running this setup for some time:
1) The standard weight flywheel is really nice is you launch the car, but if you don't plan on launching at all, I would suggest a slightly lighter than stock flywheel.
2) Compared to a JDM STi transimission that I had on my previous 05 WRX, the long gearing is not quite as fun, but fuel economy is noticeably better
3) Compared to the JDM STi transmission, driving in parking lots is much smoother, due to the lack of front diff. If you really push the car to its limits in low traction environment (wet or snow), you will notice the front diff help out. However, I felt that the aftermarket DCCD helped even more in these situations, and I believe that the forester STi trans lacks DCCD (correct me if I am wrong).
If I were to do this all again, I would have swapped my Spec B trans out when I change the clutch to get the USDM 05 trans, shorter low gear ratios, slightly shorter upper gear ratios vs. the spec B and DCCD. I would expect a less smooth ride with that setup, and slightly worse fuel economy, but that is what I would due since shorter gearing tends to feel more fun. Hope that this helps!