the opinions I've always heard is that a waterbox doesn't add any real "seat of your pants" gains. Put the $ toward cylinder porting, cylinder boring / pistons, etc.
Or, for less money, go for a lightened flywheel. I know there's debate on here about this too, but I can tell you for sure: if your ski is set up correctly, then a #Zero lightened flywheel makes a noticeable difference. (If you can't feel the difference your ski needs to be tuned better.
