I came across some info recently that may be of interest to MY99 owners , or in fact anyone with STI-type 4-pot calipers.
Strange as it may seem, toyota Landcruiser [ MY90-on] front pads are the same fitment.
Mintex make a pad for these, that whilst not in one of their 'performance' materials, when you consider the weight of a landcruiser, more than adequate for a road-going Impreza. I have a set of these on my car[38 quid] and am perfectly happy with them after 500 miles. Probably not the kit for track days, but at least as good as standard Subaru pads. For anyone who is heavily into track days, they would provide a cheap back-up in case you completely **** your brakes at a session. If you shopped around it's probably possible to get Landcruiser pads in different materials, which may not be as heavily marked up as Subaru stuff.
Mark