All of this is based on the assumption that you currently have 18:1 gears.
You will get slightly better trigger response; theoretically, your ROF should go up to 19.7rps (17.5x18/16). However, stated gear ratios are not always 100% accurate.
If you want to go a little higher and still retain good trigger response (although not as good as with the ZCI), I would recommend the SHS High Torque motor which, according to the stated performance, should get you up to around 21.5rps.
Of course, all this depends on other factors so any figures given are, at best, approximate. Shimming and motor height can both make a difference.