You would be better off trying to get more/better light if you possibly can. Anything you spend on a light will almost certainly come back to you in only 1 grow with increased production.
Lighting needs are figured (roughly) as follows: 3000 lumens per sq ft of blue spectrum light for vegging and a minimum of 5000 lumens of red spectrum light for flowering plants. So, right now, you are probably fine as the vegging plants are a bit more tolerant of underlit conditions than flowering plants and you are keeping your lights nice and tight to the canopy so you are utilizing your lighting to its best advantage. Your plants do look happy! However, when you get ready to flower, this is only enough light for a little over 2 sq ft.--barely enough room for 1 plant. Your yield is directly related to your lighting.