Dave,
I have the sti 30mm lowering springs on my sti 5 - I don't think you would want to go any lower. The MRT springs look like copies of the sti items - albeit in yellow not pink. They are probably ok though, a quick e-mail to Brett at MRT would secure more info I am sure.

for reference - the sti 'pink'uprated springs are :

part no: ST2040055060 - front (3.5 kg/mm)
part no: ST2230055100 - rear (3.1kg/mm)

the cost is 7,000 yen each (in pairs) - so circa 28k yen = £180 for all 4 plus shipping.

sti also produce 'gymkhana springs' which are black with pink spots - not the same thing at all.

the sti items make the car visibly lower (the std fit sti springs are already lower than some imprezas)with very little room in the wheel arches - ride is not too choppy - though I am a firm believer in 16" wheels - if you have 17" wheels I think things would be somewhat uncomfortable!

The sti 5 does at least have the uprated dampers - without these, uprated springs would cause havoc - as witnessed by a fair few uk car owners who fit eibach springs without changing dampers.

alternatively you could maybe find someone who will supply the 'p1' springs as fitted by prodrive - these are probably a little more forgiving when matched with 17" wheels -P1s also have the standard sti 5/6 dampers - just different springs - they aint pink though ;-)

Paul W