Hydronic in screed -> insulation underneath necessary or no?

Currently renovating, and our main lounge is 5m x 10m, with a cathedral roof peaking at 6.5m high. Currently have tiles on concrete slab. Room is very hard to heat. I am currently adding R2.7 wall insulation and replacing doors and windows with double glazed. Roof is NOT insulated, and cannot be. We have a 15kw ducted electric system, and as part of our renovation we are removing a 30kw ducted gas system ( due to ugly ducts running everywhere, not hidden, as floor is slab and roof has no crawl space)

Thinking about adding heat pump hydronic just in the main room. Currently tiles and screed are about 25mm high, and preferably I would not like to raise the floor too much.
I see there are various slim-line hydronic systems basically designed for this, eg:

https://dpphydronics.com.au/underfloor- ... m-renotec/

which claim around 15mm for the hydronic, then floor on top. Tiles would add around 15mm, which would work, or we are also thinking maybe micro cement, which can be quite thin.
Wondering how important it is to put an insulating layer between the slab and hydronic system. Most of the install manuals have the plastic hydronic placement mats being bonded directly onto the slab. Is this because the screed and micro cement will naturally heat much faster than the concrete slab? I know we have no insulation under the slab itself, so I was wondering how much benefit, both economic and thermal (i.e heat up speed) we would get from putting a layer of insulation in. Thinking maybe 6mm WEDI boards?
https://www.wedi.net/au/product-systems ... ing-board/ , which advertises a R value of around 1?