My brother weighed his engine with a stock pipe prior to putting it in his sxr and I believe it was +40 lbs plus or minus a few. I believe 750/ 800s are around 85-90lbs with dual carbs and manifolds. If you do go this route, get a carbon or fiberglass hood. That will cut 10lbs or so right there. Then add tubbies to help offset the increased weight. You can even blister the hull underneath them for more pipe clearance. Those together with a shorter ride plate that still grips and you'd be within a few gallons of gas feel wise, which should be hard to feel. Every conversion ski I've tried with a heavier engine actually handles better with the extra nose weight. Just don't be expecting a perfect freestyle set up.