I notice the lag as well. It's very apparent compared to a 2004 S2000 (cable throttle) and moderately apparent compared to a great DBW car like a 2004 M3 or 2009 M3. Those cars have electronic throttle but individual throttle bodies which reduce time to response. It is better than a non-performance DBW car such as an E90 328i.
The FT86 throttle lag gets worse when you are coasting off the throttle at lower RPM's (i.e. 3,500rpm in 4th) and then stab the throttle. It is better when the car is revving in neutral or when you are driving it hard. When you are on the throttle halfway and then press the throttle fully, the response is fast.
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CSG Mike says the lag is there for emissions reasons and was able to be tuned out of the 06+ DBW S2000. Let's hope the same is true for the FR-S/BRZ.
You wouldn't notice the lag unless you were looking for it on a test drive, but as an owner, it is pretty apparent since you can duplicate it in very specific situations (coasting down in gear).
The fix is not just changing the throttle ramp profile (that's not what the lag is from) ala "Power Mode" in an M3. The fix is actually reducing the latency from pedal press to response. There is a big difference in the two changes.
I hope a tuner like Visconti or FA20Club can comment if reducing the latency from throttle press to response is possible.