I just did this mod on my wife's 2011 Forester Premium and not only did the output jump visibly and the color get less yellow, I took a measurement with my handy dandy luxmeter and got these readings. At approximately 1M, the stock H11 bulbs = 15K lux, H9 = 22K lux. That's a significant 47% increase in peak brightness. Don't have to squint to see that. This was using bottom of the ladder Sylvania H9 (made in Germany) bought at the local AutoZone for $15 per.
Trimming the plastic tab was relatively easy. I used an X-Acto knife and a 1/4" chisel. Trimming the metal mounting tab, that was a pain.
I'm very pleased. Now just waiting on the HIR1/9011 for the high beams.