With 1 plant and that short of a veg, same veg time I go for. I doubt it'd really make it quicker, plants so small I don't think it can do anything with the additional light being that young. If it was a longer veg I'd def go with it, but not that short. I've vegged with 126w of CFL's and a 250w MH, for 4 weeks each, and didn't see a change. Obviously as soon as you go 12/12 go 400w.One plant (was planning 3 but I screwed 2 seeds for some reasons so I only have one left, thus why I want to speed up growth, as I want to achieve my harvest before it gets too hot where I live).
I usually grow 2 plants in this tent, for 3-4 weeks of veg to get the tent filled, but I usually do mainlining. This time I won't, not enough time.. But if with a 400W I'm able to get the same plant size as a 250W but with maybe one week less, then I'll go for a 400.