Why get rid of the 70?? A 70 on a well tuned combo is enough turbo for high 9's at 27+ pounds of boost..and lets face it, if you want to go fast, you will have to put some boost in it.
Some guys seem to get caught up with the bigger is better or running a huge turbo at low boost will equal a smaller turbo at high boost. It isnt the same. Only time you will see guys flying on low boost is when they have heads and intakes that flow some air such as the guys that run actual Stage 2 heads along with a huge turbo.
Why not try to max out the combo you have.
The $1500 you would spend on the turbo could buy a set of GN1's or better porting of your current heads or some bullet proof trans parts. Not knocking you, just curious about the reasons for changing.