Snipers are rarely mobile, so that might be good for getting them to leave their initial location, but after they get enough cover and/or distance, that sniper is basically no longer a factor. This might be a sniper with a jetpack or something for all I know, but when I hear "sniper" my mind jumps to someone on the stomach on a rooftop with a rifle. One sniper will probably not keep them continuously moving, just on their own. Lots of snipers can easily just turn into basically GM fiat "your characters all die now", so the answer probably shouldn't be to just having more and more of them for the sake of keeping characters running.

What about the corpos? What about the gangers? Are they on motorcycles & helicopters? Are they using close range weapons? Long range? Bazookas?