<< I have a piston pump and pressure tank water system. It provides about 40-50
psi pressure. I'd like to increase this to 60-70. >>
You are operating a tat on the high side of many well systems. The trade off
for higher pressures is always less longevity for system componenets. It's like
you won't get 100K miles out of your Honda at 100 MPH. Or if you do it will be
one poopy puppy.
<< Would a new and bigger jet pump do the job? >>
Maybe, but if you need better flow obey the laws of physics and go for larger
diameter piping. Consider discussions with both plumbers and water system
experts in your area to get the best short and long term benefits. Good luck.