If you don't need Starch (for some reason) you can replace Peas/Rice with Lettuce, and combine them with both GM Tomatoes and GM Onions. So Peas and Rice are the most efficient food producing starchy plants.
IDK which vegetables, but I assume they are probably Tomatoes and Lettuce/Onions. As was discovered by /u/user_number_666 Rice does combine with some other vegetables to produce Salad. Hence Peas are the best producers of Starch among all others. Peas and Rice are equally efficient, but Rice does not combine with Tomatoes and Lettuce (or GM Onions), whereas Peas combine with them to make Salad.
Lettuce is the second most efficient vegetable, until GMs become available. GM Tomatoes and GM Onions change things, they are the most efficient vegetables (although Lettuce is more efficient than Onions this happens because of growth speed change and GM Onions double the efficiency compared to Onions). 2 parts Peas, 1 part Tomatoes, 1 part Lettuce.