It really depends. They're not really great bang/buck by themselves. I would not waste my time with a UEL tubular manifold; save your money and stick with OE cast manifold if you want to leave power on the table.
You're looking at $1250 for the KillerB EL manifold, $50 for gaskets, and $500 for calibration. $1800 is quite a bit of money. You could do a turbo upgrade (e.g., Blouch 440XT) for around that price, and get significantly more power in the upper RPMs. The EL manifold will broaden the power curve over the entire RPM band (midrange most impacted), where as the turbo upgrade will make 50+whp more up top and shifts the powerband higher into the RPMs
If you are already planning on getting a recalibration (e.g., going E85/FF), an EL manifold is a great addition to your stock turbo setup, and would be beneficial for future turbo upgrade(s), should you decide in the future.
As for the "sound" being the reason you want to keep your UEL manifold, don't let it be. Your car will sound more like a Porsche than a Subaru, but you won't miss the "rumble" after you see/feel what the new manifold does for you in the powerband. I made the switch 8 years into ownership, and my only regret is that I didn't do it sooner.