Dave.

This is how I understand it.

Sparco top of the range Kart suit

CIK-FIA Level 2 suits (Karting) are only tested to 100 degrees. But are mainly tested for abrasion resistance. i.e if the kart turns over, or the driver falls out (rare of course)


Sparco FIA approved Race suit

These are tested by actually putting a flame on to the suit. Then measuring the effects afterwards.

FIA document see 7.1 onwards (If you're that bothered )

I would question 2 other things you;ve mentioned. Whether anyone can apply enough "flame retardent coating" to a race suit for £13!!! And secondly I think that a Kart suit whilst better than t shirt and shorts may actually be worse for you if it did catch fire as it would probably be very difficult to remove quickly and MAY melt anyway. Taking your skin with it!