These are inflating life jackets. I use one on a 40 foot yacht when I am working on the bow. They don't provide any flotation until you deploy them. They are pretty compact and I don't notice it over the top of my wet weather gear. The one I use also has a metal ring for a leash for ocean racing.

When they inflate you get a big orange or yellow balloon that pops out of the velcroed cover around behind your head (looks kind of like a Go Joe or the plane life jackets). Apart from providing buoyancy and a means of supporting your head and neck if you get knocked out I think they would be a real pain in the arse to swim a lot and sort out kite gear. You wouldn't be able to duck dive with it on.

The one on that website looks like an automatic unit - it will inflate when it is immersed in water - some use a dissolving tablet and some use a pressure switch to activate a CO2 cylinder (like a soda bulb) You would need to get a manual inflation PFD that lets you tuck the handle away, as you wouldn't want it catching on something and inflating accidentally.

One of the wetsuit majors now makes a big wave wetsuit with the same design principle as the inflating PFD - if you get held down for too long you pull a cord and the pfd inflates out of the wetsuit around the shoulders.

I think you would find it unnecessary - your wetsuit and harness provide a fair bit of buoyancy. If you are concerned about your safety in big waves I'd be taking up swimming to make yourself stronger in the water.

That's a great idea! Especially if they can hold up to typically kiting abuse (harness, high impact from failed 'sent jumps etc. )