Yes...but by a very noticable amount? Probably not...
First off, you should see if your system could run in duel channel mode. If so, yuo should try to get more identical RAM rather than a single stick.
As for just runing one or two unmatched pairs, you have your command rate(1T 2T). Many boards run in 2T anyway, so there wouldn't be any difference. However, if you drop down to 2T, then you might have a slight decrease in performace.