No test data for it that I can find. And I think it is the only booster tpye chair available as Isofix.
We have one, and another on order. It is really sturdy with good side cushioning. It comes high upt he back and has 2 seat belt positions.
When we got it is said from 9 months, but following UK guidelines it would be more likely from about age 3 years - same as most booster chairs.
We got our first one nearly a year ago now, when DD just turned 3 (or just before?) Have been really impressed with it and would (have) chose it over any other non Isofix chair. the fact that it fits using Isofix IMO means it must be safer, esp for head on/rear crashes, when a normal booster chair could be pushed forward and from underneath the child.
Non Isofix boosters fit just using the adult seat belt - it is routed from seat loops at the door side, and under the arm rest at the centre. The Jane Indy Plus has the additional security of the Isofix, anchoring the base part of the seat to the car structure. You then fit the car seat belt as before. So you have extra secuirty, not less nor not just alternative.
Sorry for blathering on - we are totally sold on Isofix!