I'm guessing the weight difference between the 6cyl (assuming a duo-prop drive) and twin 225's or 250's is probably going to be 200lbs to 300lbs (the diesel being heavier). But since the weight is further forward and not hanging off the transom, I wouldn't think you'd have any scupper issues, or otherweise, due to weight. But you can probably google for weights easily enough.
The duoprop will make low speed handling and getting onto plane much better than a single prop would. I used to have a 28' Sea Ray with a single Bravo 3 (same as Duoprop). I could maneuver that boat as well as most could maneuver an 18' runabout.
You're probably right that you'll lose some top end performance. But I would think your low to mid-range performance would be at least as good as twin OB's, with less fuel burn, of course. You may be able to get performance numbes from Grady.
If you go forward with this, make sure you get a survey done by a mechanic that is well versed with that engine. You don't want the average surveyor doing it.
Best thing? Give 'er a ride and see for youself!