There's comparative testing done by folks on the Modern Vespa site. I actually ordered some (16g) direct from a guy (user name Dr Pulley) on that site. Felt a bit dodgey, but I got them through quickly and they look good *shrug*.
Might well be trying them this Saturday if Cheeky has the time after my first service and a few other things that have come up this first week with my new Fugly

It's my (limited) understanding that the weight simply effects the speed with which they are thrown out so the time it takes for the clutch to engage. I suppose if the clutch engages too early then performance might actually suffer - I got the 16g weight because posts/tests on the MV site indicated it was a slight improvement over stock for 'willingness' to accelerate, but I mostly want them to just smooth things out. Supposedly things are less 'grundly' (nice precise term there!) with any Dr Pulley sliders.