I think they will weigh in about the same...we're talking hyper cast pistons; both are "stock". The only difference is the LP pistons' dish relief is only half as deep as the normal gasoline one. I can send pics, as I have both (I have an extra set of +.060 stock pistons)
The LP pistons aren't going to change your rotating mass, just up your compression a little.
If your looking at increasing the RPM capacity of the bottom end in a 292, you better review Santucci's book thoroughly. Your looking at lighter flat top forged pistons (starting at $100 ea?), upgraded rods, lightening the crank, etc.