Replacing my bulb and I have a variable wattage ballast, considering a 600w lamp. The 180cfm fan that exhausts the space keeps the temp about 5º above ambient room temp with a 400w lamp burning.

How much can I expect the temp to rise if I go with a 600w HPS?
The grow space is only 2.5ft x 2.5ft -- 6.25 sq. ft. Is 600w overkill?

Use a vented sealed hood and you can eliminate most of the heat from the lamp, as long as the ballast is ALSO not in the growing space. If you don't put the heat into the space, it won't have to be eliminated.

You can add as much light as you desire as long as you can still keep the temps under 75f.

I went from 600w to 400 over my dwc veg tubs, and saw no reduction in growth rates. More light only = more growth if everything else is dialed in perfectly to match it.

Harvey, precisely. Everything has to be dialed in properly. More light = more growth/bud only if all other factors are kept in alignment. ie; heat and humidity control, air movement, etc.

I recently (due to summer being harder to cool my room) put 2 600 watt hps to the side and am only using 1, 1000 watt hps in flower. During the winter months, I can run all 3 and not have heat issues but in the summer in Nevada (brutal)I roll it back. Funny thing is that due to experimentation etc, I have figured out how to obtain a fine harvest with 1, 1000.
After having beat my head against every wall one can find to bust it up against, I can give ya'll some advice in this department.
Before adding more light, build up your infrastructure such as cooling, hot air removal, and maximize your potential yield with the lights you have before getting even more. I mean really, think about it, someone already has 1000 watts and cant maximize that, so they think the answer is more lights, they go get the lights but fudge on an indoor portable air conditioner. The end result is that they got a bit more bud but the plants were skimpy and there electric bill increased dramatically due to the central air running 24/7. If they were to do the math, they may even find out that their cost per gram actually increased rather than decreasing. So, the grow cost them more, they got mediocre bud that made even less $$$ thus causing their bottom line profit to decrease rather than increase. Not good business.

In summary, never add light without firstly having built up your infrastructure and maximize your yield potential before multiplying your lights. Then when you do it, you will also double your yield and lower your costs.

Now in answer to the original question. With the additional heat, is your infrastructure set up to handle the new heat load and keep you under 74f at all times? If so, then add the new light. If not, then add better infrastructure and then add the lights later.

Thanks for the replies, but I'm not sure how any of the posted info answers what I thought was a fairly simple question:
How much more heat does a 600w produce vs a 400w?

and you never said what ambient room temps etc are,ie; you did not give us enough information to properly answer that simple question, therefore we decided to cover the topic from a to z just for the fuck of it.

In answer to your question,I refer to my last post, last paragraph.

Well, Jiff, it's 150% more heat, obviously, but how the hell are any of us going to be able to tell you how many degrees your room is going to go up?

And that wasn't the only question you asked, you also asked "is 600w overkill?" which is why I wrote about dropping wattage with no loss in growth.

The more watts means more heat..
As long as you can control the heat build up and keep temps to around 75 degrees.. You can run as much light as you can..

I run 1000 watts and its a bitch here in California this time of year. Have to run a small AC unit during late spring and most of the summer.. If I do not do this??? I get Hermies all around.. The Strain makes no difference.. All do this..

You can run more light, but there is definitely a limit to this. In my garden right now, the parts of the plants that are too close are light colored and crispy looking, and the parts of the plant that are farther away are darker green and healthier looking. More light does not always equal better. I'm actually considering dropping some of my 600s to 400s just to see what it does.

It's not all about heat. It's about what the plants can actually use. With good air movement, co2 and good light, usually something like your root zone or your nutrients not being dead on will be the limiting factor. That's why I mentioned how I went from 600w to 400 over 2 of those 27 gal containers I use for veg with no loss in growth rate. The 600 was just a waste of electricity.

And as far as heat, that's not that simple either. I noticed an increase in metabolism and growth with increased heat, up to where I am now, which I don't exceed for other reasons. I haven't experimented with temps consistently over 85. I'm currently usually at 81-82 degrees. 75 degrees, 70 degrees, 65 degrees, 60 degrees, I observed less and less growth all the way down to about 50, where the plants completely stopped growing. (basement in the winter) My friend in CA said his outdoor plants grew fine at 115, he just had to water the hell out of them. You always hear they stop growing over 85, well, there you go.

Best answer, as always, experiment.