Modifying the suspension to handle the weight is something you should be doing. What you should not be doing is adding 150# of weight that high over the vehicles roll center. This is the pivot point where your weight and suspension fight the ground. This is mostly felt side to side but also effects front to rear. The longer the distance above this point, the more leverage working against the suspension. Modifying the suspension only modify's the weight carry. The only way to properly do this would be to lengthen and widen the stance or lower the vehicle. Anything we put on our roof will act the same. Next add in tire/suspension lift and the ground point on this equation changes also. Jeeps are short and narrow. My Cruiser is much longer, slightly wider and much heavier. I wont feel the effect as much if I put something on mine compared to a TJ.
People do these things all the time with no knowledge of what they create. Thousands of people run things on their roof without any issues. As long as you don't get too technical or get into a high cross wind, you never notice. AS the OP pointed out, sidehill's can be scary with an RTT.