go visit this page above you will see that when you shift at 13000rpm or little below there is 11,5 hp difference. At the top end there may be around 7 hp however as we all know we dont shift at the peak we shift it after the peak so for the next gear we get a good hp.
so imagine what difference 11.5 hp will make when you shift at 13000. furthermore this is without a powercommender. I believe that with a well tuned pc you will get more out of it.
that will make a lot of difference. but if you dont want to spend money thats up to you.
I also owned zx 7s. 89-90-91-93. Thats the bike ı love most. However it is carburated and I do not remember that ıt has air fuel sensor (11 years ago) I also remember that I used to jet it with bigger jets at the inside cylinders than outside cylinders.
you mentioned that your friend went 2 down from the rear. All the bikes out of the factory are tall geared to compensate the back wind and downhill factor in some case. So going down 2 from rear will not serve for top speed because the bike will never have enough hp to pull with this gearing. Therefore without any mods going 2 up from rear will give you what you need but if you have mods and extra hp than stock since you have more power maybe 1 up from rear may be enough.
you may try to disconnect the pc and run with stock ecu and just the pipe will give you good power ı think. because a wrong map could put you down of your stock hp.
ı also dont like to spent to much money but in my country we pay 16.000$$ already for the bike and I can not stand to get passed by another bike and I also ı dont like to mess with the exup system but a full akra makes a lot of difference. you feel the extra hp at the 12k 13k rpm range and thats what pull the bike inch by inch from the other and for myself the satisfaction worth any penny.