What are less stims? Not sure where you heard that you will get the same density with T5s as HPS, as you will not--not even close. And the buds will be lighter and airier when compared to HPS. And you will need quite a bit more watts to equal the same usable light. In addition, you are not going to want to veg with red light (HPS). You might want to do some more reading up on lighting needs for cannabis. You want something in the blue range (6500K) for vegging and something in the red range (2500-3000K) for flowering. And you want your strongest light for flowering. So running HPS for vegging and T5s for flowering is backwards. I run T5s for vegging and HPS and LED (2 separate closets) for flowering.
Not sure what you mean by massive, but 800W of T5s and/or 600W of HPS is not enough light for a massive grow. You need to adjust the size of your space to the lighting you will be using. You need a minimum of 3000 lumens per sq ft for vegging and 5000 for flowering. We do not use wattage as different types of lighting put of way different amounts of lumens per watt. For instance, most MH lighting puts out 90 lumens per watt, T5s from 92 to 100 lumens per watt, HPS 125-150+ lumens per watt.
You have mentioned nothing about ventilation, but it is almost as important as your lighting. Do you have the equipment you need?