Clearly most of the critical reports suggest that the Rascal's stock motor was inadequate for the purpose but your PSJ 800E will certainly be a dramatic improvement. You can go to MotoCalc (30-day free trial) or
http://brantuas.com/ezcalc/dma.asp (free for ever) to get estimations of thrust with different props/batteries etc. As long as you realize that the accuracy of these predictions is a matter of considerable debate - they do provide something to look at.
Cheers, Phil