Did you find the IGT S+ Bible ?
Go to the Home Page, Ricks FAQs and its under there. It will tell you which chips work with which theme.
All chips work in all machines - to change themes you change the chips glass and reel strips.
To get to the heart of your question - changing from a 92% chip to a 96% chip won't change your "hit" percentage by any noticeable amount.
Assume a $1.00 machine and a 92% payback that means the machine would hold $80 out of $1000, a 96% machine would hold $40 out of $1000
3 x double bars is a 20 coin pay back. 2 extra hits in this range would be the difference in the overall payback on 1000.
Would you notice 2 extra wins within 1000 spins.
Payback Percentages are based on 10million spins and there are sometimes long droughts - Ie the machine paying back 20% and other times paying back 160%
I might suggest you roll your coins - and hand those out. People then feel a loss when they go through their roll vs just having a cup full of coins next to the machine. They will come back and look for more.
Lots of marketing studies done on how limiting quantities drives demand....(and we are not just talking about loss leaders).
You might also go with tournament chips for your machines. You can set a time limit or a spin limit - these are fun and almost every spin is a win.