What are the options for radiant heat with a slab on grade?

Radiant heating is very comfortable, and there are really only minimal differences between the comfort that different systems will deliver. I would base my choice of systems mostly on cost and ease of installation, along with design considerations of the house.

Warmboard sounds like a system worth considering if you desire radiant floor heat in upper floors, as are other products, but to answer your slab question, I wouldn’t be using it in basements. You simply need to embed tubing directly in the concrete, no need for additional products.  We have lots of pages on Radiant Floor Heating, have a look here and you will probably find all you need - 

Choosing a Radiant Floor heating System

I took a look at the  Warmboard website, and a few things jump out at me right away that I wasn’t overly impressed with. Any company would be foolish not to highlight their advantages while at the same time making little mention of their disadvantages, that’s business. But coming out of the gate with misleading claims makes me wonder what other questionable things may be going on.  

1 – “Our patented design is the industry-leading low-mass radiant heating panel”.

While it is obviously true that it is low-mass, I wouldn’t necessarily consider it an advantage. A lack of thermal mass means the system will heat up more quickly than a system that has some mass to it, the flip side is that it will cool down quickly as well. In some situations that may be an advantage, but the opposite is true as well. Read more about thermal mass in homes here.

“Warmboard provides the fastest response”  Yes, a lack of thermal mass would mean it provides a quicker response. Is that a good thing? As mentioned above, if it warms up in minutes it will cool down in minutes as well. So, in the event of short-term power outage (think a day or two) you have no reservoir of heat to keep your home comfortable, which is one of the things I personally like about radiant heat. 

''Warmboard is a giant leap forward in the evolution of radiant heating. Every aspect of the design and manufacturing of Warmboard has been implemented to maximize performance.''

I don’t see any leap at all here, I just see another fish in the pond.  When you run heated liquid through tubes in a floor, the floor warms up. There can be some minor variances in the comfort they deliver based on a few design considerations, but there is nothing ground breaking here. 

“Our high conductivity brings the lowest water temperatures, decreasing energy use (and energy bills)”

Nope. And this is the part where I become suspicious, when they make claims that just don't line up with the laws of physics. They may have lower water temperatures, but if there are less BTUs being generated, there are less being delivered into the house.  Injecting heat into a mass of concrete (typical radiant floor systems) means the heat is in the building envelope and will be delivered eventually.

To put that claim in context - you will use less gas by the minute when driving at 60KM per hour rather than 80KM per hour, but it will take you longer to get where you’re going. Since you are using ‘less gas’ for a longer period of time are you actually using less gas? Nope. The claims on Warmboard's website make as much sense to me as a car salesperson telling me I'll get there just as fast at 60Km per hour in their car as I would at 80KM per hour in their competitors car. 

I would price it out compared to other systems, and mostly watch an installation video and see if its a system you think you'd like to work with. but don’t be dazzled by their energy efficiency claims.  It’s a fine product in the right application, but there is nothing all that new about it as near as we can see.