So will there be a significant drop in volume since Kraken and Bitfinex as of 8th of august cut their services in New York?
It really isn't difficult to get around a "location ban" especially considering that bitfinex (and maybe kraken) allows you to trade without verifying your identity. All you would need to do is create a new account while using a VPN gateway outside of New York and you will be able to use bitfinex worry free.
To answer your question, I would doubt that either exchange would see any kind of significant drop in trading volume. In addition to the fact that people can get around a "location ban" (see above), even though New York City is a financial center for the US (and the world), many people who work in NYC actually live across the river in New Jersey, and would not be subject to such restrictions. I also would not think that the trading volume from NY residents is all that significant in the first place.