The differing ratios work better on different engines depending on gear change points and so on, I onyl had this explained to me the other week when i was looking at what ratios to go with, i'll try to explain but i may be waffling haha.
To start with you need to look at the differences with the engines, 2.0 is a more revy engine, so typically nothing happens till 4-4.5k rpm and pulls up to 7.5k on a jdm sti, so its typically a slower spooling setup and pulls harder higher up the rev range.
2.5 prefers to have low down load and produces alot more torque, so tends to pick up from 3-3.5k (varys depending on setup of course) but once you hit 6.5k it tends to drop off the power band. The extra torque the 2.5 produces can mean if the ratios are too short, the turbo will never make full boost because the engine is pushing hard enough so that the turbo can't fully spool before it runs out of revs. aka you end up with 3rd gear pulling harder than 2nd gear.
A shorter ratio means your loading it at higher revs so suits a 2.0, where as a longer ratio will building load lower down the rev range and suit a 2.5.
So that said the choices i had were,
1, UK Hawk 6 speed DCCD, ratios designed to work on a 2.5
2. JDM 6 speed gearkit DCCD, ratios designed to work on a 2.0
Given i'm running 2.5 and its already kicking out 380ftlb at 3.5k on the dyno (road is quicker spooling than on a dyno), that figure is only going to go up with the boost increase, so short ratio gears won't be as critical because i will be changing gear more than on power if they are too short hahaha.
Hope that makes sense lol