I have a friend who swears by 4banger tranys....it's kind of funny but every V8 T5 he gets he blows up....but he has yet to blow a 4cyl T5. I can't deny that they are weaker, espcially the input shaft, but i also stood and watched him bust off easy mid 13s power shifting a 4banger T5 and nothing bad happened. It isn't as strong, but if you know how to shift and your car is still mostly stock then it'll survive for a while. We use a bearing from a Mazda truck when we swap them over. I'm not sure about part numbers but the one above i'm sure is probably right or he wouldn't be posting it.
I wouldn't dare run one with slicks or a power adder though!!!