I'll throw this into the debate. I've had the Fazer now for 8 years, I had a PC5 fitted and set up on a dyno to maximise performance, Scorpion system and decat link pipe. I still found (a common problem I believe) that when closing the throttle from wide open, then opening it up again, the response time was not immediate, possibly more noticeable to me as I do several trackdays a year on the Fazer. However, the last time I filled up, I sought out and used BP Ultimate. Although I have not changed anything else on the bike, the problem has 95% disappeared, and now picks up smoothly from closed.
The ECU cuts the fuel on the overrun and it takes a while to build up fuel pressure again when you apply the throttle this causes the lag you describe, akin to turbo lag in years gone by. There is a Fuel Cut Eliminator available from Ivans Performance in the states which sorts it
https://ultimatemotorcycling.com/2009/10/27/2010_yamaha_fz-1_abrupt_throttle_fix/
