Is this what you are looking for:
I do not think there is any performance difference between the 2 , they are both 10 Gig - sr.
I think the only difference is the form factor, X2 is the wider transceiver (like a GBIC) and i think it should be SFP+ which is the size of an SFP.
Please correct me if i am wrong
Yep, All i could find was the only reason to have the transformer was if one type was not available or the client wanted to have the same throughout their solution. Went through all teh data sheets so like you i concluded there were no performance or latency differences between the two.