Thinking of upgrading veg bulb

I'm growing 6 plants in a 120cm square tent. I have a 250W MH bulb in a euro reflector about 15 inches above the plants, but the real distance is further than that because of the width of the grow area. Temp during the day (lights are on 24/0) is about 25C / 78F. Growth has been slow, plants are about 20 days old and only just on 3 leaf sets inc cotyledons.

Would I benefit much from switching to a 400W bulb and setting it slightly higher to allow light to hit all the plants more evenly?

Would it increase growth rate?

Would the temp be too much higher?

It will be expensive power wise to do this so I want to be sure I'm doing something beneficial before throwing my $ at it.

Hey dude, I veg seedlings under a 2' T5 with four tubes. From popping their heads through til about five nodes and they grow rapid.
At five nodes i top them and put them in 3L pots and veg them for a couple of weeks under a 250w hps. Then into 3gals and under a 400w.
The 250 might be small for your tent but if the plants are in small 3L pots then packed under the light their getting what they need.
If however you've got little plants in big pots nowhere near your 250 then yeah put a 400w bulb in they'll need it.
I always pot up and change the lights because the plants grow so much quicker. Put a little plant in a big pot and it's too easy to over water and then you get a plant that just sits there with wet roots doing fuck all!

Yep, little plants in big pots. If I'd known what slow growth I'd get I would have used smaller pots, but digging them out now would cause more stress than good.