Modern Vespa

I own a 2020 MP 500 and I decided to design and build a cargo rack, which has helped a lot carrying boxes and luggage, which is excellent for long trips and most important a bit aerodynamic, no need to carry saddle bags and because I am able to weave thru traffic in Southern California.

That looks pretty good! Very nice modification that looks professional in build quality.

How does it affect handling on some of the twisty roads laden vs empty? I suspect you cannot lane-split or filter in some of the tighter scenarios.

It was done using plasma welding and cutting, and i can still do split lanes and drive thru traffic, because the cargo rack is using the natural lines of the vehicle, above the seat, no problem whatsoever, another good thing about it is that the weight is evenly distributed on the rack and supporting beams.

How much weight can it carry and how much does the rack itself weigh?

IIRC, Piaggio states a max limit for the top box carrier of 10kg (22 lbs US).