I know you didn't ask me WhiteWolf, but here is my $00.02
I put the 18mm bar on my Forester before the strut/spring swap. I noticed an immediate difference. Much of the understeer was gone and turn-in was immediate. No oversteer, either. It made a big difference. There is also a 19mm bar from Cusco that bolts in, if you want just a little bit more neutrality. That will make front and rears equal. I didn't go for it, because it was gonna cost more and I had a budget to stick to. (I still went way over :lildevil
I know TRi Tuning had the Cusco 19mm bars, you could try them, or just go with the 18mm FHI bar. I haven't tried it, but I would think the 20mm would make the Forester a little too tossable for my tastes.
Anybody?