When racing all classes it's about trying to give yourself a competitive advantage. Fitting square or round packs makes no difference. There are always better batteries round or square.
It won't change mini racing, when we did it with M03's you could fit the latest / greatest Nimhs. It's a daft argument.
As noted the big expensive square packs only give a bit more power which can easily be negated with the extra weight and the fact that the fwd mini can only grip a certain amount.
I run spec class 17.5 and good lipos can be cheap, also there are more options, it's a better scenario than hunting for round packs. Mini's were a popular second car/class but once we got lipos etc they warned in popularity due to them not taking std square pacs. This update has piqued the interest of a load of guys again as we can just run these with 21.5's and no boost esc's.
The biggy for me is droop, easily one of the best set-up options on a car, and now much easier to do without having to rebuild shocks. This will give people an advantage as you can tweak it easily over the course of a race night along with the camber etc to optimise the handling of the car.
I'm getting one, been waiting for the mini to be appealing again