I've owned both...used to have a 2008 IS-F (sold) and currently a 2016 GS-F

 

I've been driving them back to back for a couple of weeks now. Issue #1 - the kickdown switch in the GSF is too stiff and artificially limits WOT. I chopped the springs in my ISF's kickdown switch and it is no longer noticeable when I go WOT. I think I need to do the same thing to the GSF to be fair. Issue #2 - the GSF seems so much more gentlemanly than the ISF. My ISF throws me back in the seat with an attitude while the GSF squeezes me back. It's just not the same, but both cars leave the average driver in the average car totally for dead. Issue #3 - I am still sorting out all the options on the GSF, and just today I figured out the mirrors will not do stupid things in reverse if you pair the key with the personalized settings. I've bee suffering because I normally set my mirrors to zero overlap, and the damned things go crazy wide if you don't have a key paired to the seating preset. The ISF does not do this. Pissed me off to no end at first because I always back into my garage, and more often than not, back into parking spaces to ensure I don't scuff the carbon fiber nose (OK, I do the same thing with my ISF too).