We are currently using a metal halide on tomatoes etc and one of the plants is destined to go outside. Careful to keep the plants compact and see the link below for some techniques. I am not sure of the benefit other then a more mature and hopefully healthier plant to start. The MH will not mess up flowering later. Transplanting really big plants is sometimes hard and if you shock them in the move it may stall the plant growth and negate some of the benefit from raising a large plant. We are keeping ours in a pot to move to the patio, to avoid transplant issues on a plant that will be 4-5 months old. Good luck and it is a lot of fun.

Here is a link that might be useful: Inovative tomato grow