I am right handed but occasionally shoot off my left; I have a SFIII and find it a bit scratchy on the cheek (with or without shells), but not a huge problem. My buddy who is a pure lefty has a SFI and it does not bother him. Neither of us do quick reloads, so as far as using the stock for quick loading I can't say.
You could consider the SF II or IV which are the same as the I and III, but without the magazine tubes. Of course the mag tubes are the whole reason I went to Speedfeed... if you still want to hold shells on the but, other options would include a Knoxx or Mesa setup.
edit...
To save you the dissapointment I went through, its also worth mentioning that the Speedfeed magazine tubes will NOT hold two 3" shells. They will take two 2 3/4" shells, or one 3" + one 2 3/4". Someone with a little more "do it yourself" aptitude could probably modify the tubes to hold two 3" shells, but I decided it was not worth my effort