Actually, it might be just me or what but I'd rather shift with my left arm and turn with my right. I'm still a right-handed person, it's just since I use my right hand all the time and therefore, I can do quicker manuevers with it. Shifting is not that important, I mean how hard is it to pop a gear into it's place? If it all came down to a drag strip, then you're right - you have more strength to do that quicker by part of a second. But you always can keep practicing and you'll be better with the left hand later.

As far as your comment on the why we have a left-hand drive cars that's not completely true. There was more going on than just which arm you use. Check this out --> http://www.brianlucas.ca/roadside/
http://www.2pass.co.uk/japan.htm