Coming from a few Evo 8's and Subarus with various coilovers and what-not, the 08 ISF is probably a little more harsh than a stock Evo in my opinion. On nice roads all is good. But over bumpy pavement or freeways it's a very busy suspension and definitely bouncy.
The issue is really with what you like day-to-day. I prefer a firmer suspension (but not track-ready) and really hate floatiness so I don't mind the ISF being a little annoying at times. But really that's only when I want to kick it into full Auto and just chill or I'm trying to drive nicer because of passengers.
My wife complains on occassion...although she also falls asleep most of the time when we drive back from visiting her folks. And this is on So Cal freeways so obviously it's not atrocious if she can fall asleep.
The one thing I feel the need to mention (briefly) is build quality. In nearly every previous car with more firm suspension, random parts of the car would rattle or vibrate. In the F all is silent so it's a nicer, yet firm, experience. That has been the big difference for me.
Personally, if you have the money to spend a few $k on suspension stuff to make it better...just use that money to get a newer model. They really improved things 2010+ and then you won't have trial-and-error trying to get it right yourself.
Steve // 2008 IS-F / Starfire Pearl / 65k miles / Tint: Front 35% Rear 20% / JoeZ PTS cat-back exhaust / Beat-Sonic Nav+DVD bypass / Vaistech SL2Vi iPod integration kit / carbon fiber de-badged front grille / carbon fiber vinyl on lower rear bumper / S4Play rear diffuser / Escort Passport X50 / 80GB iPod in the armrest //