Yeah in many cases the suspension on super low cars is setup for looks, so performance isn't the main priority. With the likes of weld on sleeves, the springs used are much smaller diameter, so they can sit much lower than factory perches. 30mm of extra spring length will always be good. If I was going super low, i'd go weld on stuff for sure, you can set it up exactly how you want. Using factory perches does limit you quite a bit in comparison. From the photo, it looks like you could even get more than 30mm lower if you went aftermarket weld on sleeve option?
Just my opinion though.