I use the OE diff ,just because i found out that mine (Hammad Guhman) aluminium diff is not exactly the same size than the OE associated one ,so i just preffer running that one aside that they are so cheap and i have so many spares that i can run them for years .
I said that it depends on what you do because in US you can not race with a 18 engine in a ST ,but as here in Finland i run it with the 1/8 ,no one cares about that and i can run my GT as i like it ,but to be honest ,a good 12 engine is more than enough for a GT2 or GT ,but i like to have lot of power in case ...