You may not be too early. Don't know when you put them in the ground there, but I start my plants at the end of Jan. and put them in the ground at the end of May.
Plant quality is dependant upon the quality of the nursery stock you were getting before.
Yield is primarily about variety.
By starting your own, however, you have vastly more choices on variety than you could find at the finest nursery around. You also have the potential to have more mature plants to put in the ground, which may give you an earlier yield, which will, of course, give you fruit before all of your neighbors. :D
The downside to starting your own:
Tomatoes are very susceptible to damping off disease. Proper enviroment is critical.
Temperature is also critical. Start them inside where temperature is easier to control. You want to maintain 80 degree soil temp for 7-10 days for proper germination. Try an electric blanket on a table top.
Once they're producing their 1st real leaves, not just seed leaves, their pretty easy. They can go in the greenhouse then, as long as you can keep them warm, with good air circulation, they'll grow like weeds.
I do about 1000 plants a year. It ain't rocket science, and I still find it very enjoyable.