Isofix High Back Booster with harness?? Moon on a stick?

[SailorVie]

I have a 3 year old DS who weighs 17kg. His maxi cosi cabriofix has now been passed down to his 10 month old sister who weighs 12kg.
I have been looking at the Britax kidfix but was wondering if there is an isofix high back booster that reclines and that has a harness?? Am I asking for the impossible?
I am using a Britax Adventure which is crap, he has to sit bolt upright and slumps completely over when sleeping on car journeys, the car seat seems to move about too much for my liking so I'm really keen to get an isofix seat.

Can anyone advise??

In my experience all the high backed boosters use the adult seatbelt and the only recline you may find is minimal. There are however erf (extended rearfacing) seats available. They are however very expensive. In the highbacked boosters the kiddy range is very good and gets excellent safety reports. They use a large table type safety cushoon to hold the child in so they cant slump forward.

The concord transformer xt is fantastic. It it does use an adult belt though. I think the staps are only suitable up to 15kg anyway

I think britax do one with a 3 point harness. Not sure if it's isofix though.