i couldnt use my 250 watt hps because it was to hot, on the box it says it gets to over 300 degrees, i think thats may be close to being as hot as an iron, so i swerved the hps without ventilation, you definately need ventilation for hps.
I grow under 200w CFL and 100w HPS, you WILL need good ventilation (intake, fan and exhaust) I have 2x80mm PC fans drawing air into the bottom of my small grow room, a desk fan inside blowing air around and 2x120mm PC fans sucking air out from the top.
I would recommend buying a 250W HPS balast and get a 250w MH conversion bulb that will work in your HPS balast.
Start off with your metal halide bulb and then change to HPS when they start flowering.
See my grow here - https://www.rollitup.org/general-marijuana-growing/97151-northern-lights-harvest-pics.html
You should get good results with a MH / HPS combo but only if you're growing a couple of plants.
Sure you can grow great weed under a 250, but they're limited to maybe a 5 square foot (of floor space) growing area like a well ventilated little grow cabinet.
Think about a DIY cooltube.
Either that or you'd need several of them for a larger area.
They'd do better than CFL's in a cabinet.
Here is a picture of my plant grown under a 250mh. (not in a cabinet just one plant with one light on it)