So here's a gearbox:

and some info gleaned from USMB:
So it seems to be possible, just needs some money thrown at it. When it comes to this stuff I'm a dunce but I'm sure there's enough smarts on here that we could work outthe way it works is its 2 sets of gears, thats why low makes so much whining. First you have a set of 1 to 1 gears to a secondary shaft, and then it gears back across to the low floating gear, and thats where the reduction is. when in high it just bypasses straight thru from the input shaft to the rest of the floating gears. Kinda like how "hi" gear is on a normal transmission. SO in theory, you could double up these reduction gears, and get somethin crazy like 2.8 to 1. The only problem is that this would require a 100% custom input shaft, and a 100% secondary Low shaft. but the floating gear and the rest of the tranny would be ok.
Funny enough subaru transmissions are nothing like a normal tranny, because Hi gear does not go straight thru, but rather is also a floating bevel gear, just like the rest, because its the counter shaft that drives the ring gear on the differential
a) It's possible
b) Exactly what needs to be done
c) Who can do it and
d) How much it would cost