A good friend of mine is the master at using row cover to start plants under to get a jump start on the season. I have also used these on a smaller scale for corn, which does not like the cold Alaskan soil.
Anyway, I think starting tomatoes under these then taking off plastic after the ground has warmed, will affect seasonal yields by increasing length of the season, just like any other crop.
Irt covers the row, then plastic on hoops over that, about 3ft wide or so, not as tall, tension wire hoops work great, and they are super cheap, just buy a roll of wire and cut it.
You will have to use drip irrigation, or something of like to water though, at least until you pull the plastic cover off. Here's a decent example I found online.
http://edis.ifas.ufl.edu/cv201