Interesting thought.....
 

Ok we all know the usual first mods to reduce understeer (ALK, rear ARB and droplinks) but I spotted a set of bugeye WRX wagon springs on fleabay for a fiver, little bit of research has told me they are about 10% stiffer than saloon and will raise the rear ride height by about 1/2" (raising the rear roll centre by nearly an inch), so basically if you have a WRX saloon a pair of wagon rear springs (cheap enough...) would be a good way of helping reduce understeer on an otherwise near stock car all staying with 'Subaru' parts....

Not sure if this info would stand for classic, but I suspect it would....

Simon