Defi is a brand name of the Nippon Seiki company. Defi is a french word which means "challenge".
Nippon Seiki are a manufacturer of instrumentation panels for a large number of car manufacturers, including Subaru.
Their Defi branded goods are aftermarket gauges for things such as Boost, Oil Temp etc. etc.
They are reassuringly expensive because they are the dogs dangly bits.
http://www.defi-shop.com/
Oh and I have the STI Genome gauges, which are Defi items with the STI logos on them. Very nice indeed they are. I got them off ebay, but the seller has none in just now it seems.