cooker - cant help ya bro, i dont use mylar..
letsdothis - in a cab that size you shouldnt need a cool tube, but it would have its benefits in a cab with limited height esp if you're growing a strong sativa dom plant, as it could stretch alot, and using a cool tube will allow you to let your plants grow closer to the light without burning.
150w hps dont really throw out too much heat, mine are around 79degrees @ 7" away from bulb..and some of that is trapped heat under the reflector.
If you currently have the heating on, your cab temps could be noticably higher for obvious reasons, but a good 4" fan installed in the cab as an Exhaust will pull the heat straight out.
thats what i use and my temps are steady 75-77* during summer + winter.