Upgrade from 300w mars hydro TS to FC/FCE?

primary benefit will be efficacy and longevity of the equipment. it will last longer and require fewer watts per sq foot to 'max' out DLI relative to you local environment. so, if you already more than sufficiently cover your garden size, given the dimming, this will not impove quality or yield, but you'll probably save a few watts, because you'll have to dim it slightly more. i don't know what the CCT is of each, but a bit warmer is generally a good idea if it's a vege and bloom light. that's supposedly better for denser buds, but remembe genetics is the primary dictator of that and the lights merely make some small difference. max dli is primary concern. fine tuning the power to give the most while not damaging the plants is most of what we can affect. if you only do autos now, it'll give you the opportunity to do 12/12 photoperiod bloom. fc 3000 will cover 3x3 12/12 with ambient co2 easily enough.