I can't speak from personal experience on the DGV downdraft conversion (I've just used SU's and DCOE's) but to the best of my knowledge:

1) Not a performance carb - either the SU's or the DGV can cope fine with a bone stock engine, but as you start tweaking the engine the DGV will hit the wall sooner.

2) Economy is highly dependent on the tuning in either carb's case. I'd just have to give the nod to the SU's here because of several factors. One - they are adjustable so if you know what you are doing you can tune them to perfection. Where tuning the DGV involves swapping little machined brass bits - not something easily done. And I think the SU's save a little fuel in a static cruise state because they will settle into a slightly leaner mixture, where the static Weber won't.

3) The only way they could be considered an improvement to a Volvo is when you compare them to a set of work out, air leaking, maladjusted SU's.

The DCOE's are a different kettle of fish. With a pair of them there are clear gains in power and fuel consumption. >:^)