I had an FK8 for 5y and 56,000km, incl. winters in Canada. Now have FL5 for about 3mo.
Both are really great cars - FL5 is more serious in how it goes about its business. Somewhat firmer ride in all modes, but still very impressive damping quality.
Interior is a huge step forward on FL5 if that matters to you, but FK8 is very functional interior, just not as nice to look at. Bose stereo in FL5 is better than crap in the FK8.
Seats are similar, FL5 has a slight edge - both are just excellent seats!
Braking feel (for street driving at least) is similar, again slightly better on FL5.
Steering on both is delightful, FL5 feels slightly less talkative in normal driving, but once you load up the steering a bit through some quicker corners, it comes alive more than the FK8 did and feels really nice.
Main improvement is how FL5 handles its boost - it's somewhat more responsive and closer to driving a very powerful NA motor than the FK8. Overall power feels essentially the same. Impressive tuning - very easy to meter-out power and find the most traction without just spinning up wildly.
Other main improvement is the Individual mode - this alone really allows you to setup the car the way you want. I like heavy steering but at times on our roads I need Comfort suspension - I can only get this config on the FL5. AND - the car remembers what mode you were in and starts up in that same mode (except for +R mode).
^^^ Agreed about everything the above person said. The quality of life improvements are a big jump forward. Interior so much better, sound system much better, infotainment much better, honda sensing standard, Individual mode is awesome (works the same my Porsche 911 and M4 does), the motor feels less boosty and more Na linear, chassis is one notch up sportier and firmer, 19 inch wheels with 265 tires all around is more compliant, the torque comes on buttery smooth and very progressive. Looks MUCH better than an Fk8 (to me) If none of the above matters to you then save some bucks and buy the Fk8.